skyshine]

THESIS

SKYSHINE RADIATION RESULTING FROM

6 MV AND 10 MV PHOTON BEAMS FROM A MEDICAL ACCELERATOR

Submitted by

Deirdre Harris Elder

Environmental and Radiological Health Sciences

In partial fulfillment of the requirements

For the Degree of Master of Science

Colorado State University

Fort Collins, Colorado

Summer 2008

i

ABSTRACT OF THESIS

SKYSHINE RADIATION RESULTING FROM

6 MV AND 10 MV PHOTON BEAMS FROM A MEDICAL ACCELERATOR

Skyshine radiation scattered in the atmosphere above a radiation therapy

accelerator facility can result in measurable dose rates on locations near the facility at

roof level and on the ground. A Reuter Stokes RSS-120 pressurized ion chamber was

used to measure exposure rates in the vicinity of a Varian Trilogy Linear Accelerator at

the Colorado State University Veterinary Medical Center. The linear accelerator was

used to deliver 6 MV photons and 10 MV photons with several combinations of field

sizes and gantry angles.

The equation in NCRP publications (2005, 1977) for modeling skyshine radiation

in the vicinity of medical accelerators is not a good fit to the observed dose rates at

ground level or on the roof. A more accurate method of estimating skyshine may be to

measure the exposure rate of the radiation exiting the roof of the facility and to scale the

results using the graphs presented in this thesis.

Deirdre H. Elder Environmental and Radiological Sciences Department

Colorado State University Fort Collins, CO 80523

Summer 2008

ii

iii

TABLE OF CONTENTS

ABSTRACT OF THESIS……....…….…….……………………………………….......ii

TABLE OF CONTENTS.................................................................................................iii

Introduction........................................................................................................................1

Materials and Methods......................................................................................................3

The Accelerator Facility..........................................................................................3

Radiation Detection Instrumentation.......................................................................6

Calculation of Expected Radiation Levels...............................................................8

Results...............................................................................................................................10

Roof Transmission.................................................................................................10

RSS-120 PIC Calibration and Constancy..............................................................11

Skyshine Radiation at Ground Level Locations....................................................12

Skyshine Radiation Levels for Outside Locations at Roof Level.........................16

Skyshine Radiation Levels at Second Floor Corridor Locations..........................19

Discussion.........................................................................................................................20

Conclusion........................................................................................................................25

References........................................................................................................................26

Appendix A – Calibration ..............................................................................................27

Appendix B – RSS-120 PIC Constancy.........................................................................30

Appendix C – Exposure Rate In Beam and Roof Shielding Transmission Factors..35

Appendix D – Calculation of Expected Skyshine Dose Rate

Using NCRP 151 Method........................................................................36

Appendix E – Skyshine Data and Analysis for Ground Locations.............................40

Appendix F – Skyshine Data and Analysis for Outdoor Roof Locations...................60

Appendix G – Skyshine Data and Analysis for Second Floor Corridor Locations...71

Appendix H – Comparison of Skyshine to Primary Beam..........................................78

Introduction

Skyshine is the term used for radiation that originates near the surface of the earth with an

upward velocity and then is scattered back by the molecules in the atmosphere. Skyshine

radiation is of concern because it can deliver radiation dose to the public in areas beyond

the boundary of the radiation production facility, even in areas that are not in the line of

sight of the source. When the Brookhaven Cosmotron and the Lawrence Berkeley

Bevatron became operational in 1953 and 1954, the radiation levels in the vicinity of the

accelerator facilities were much higher than predicted. This was due to neutrons that had

been scattered by the atmosphere (Thomas, 2001). Also, as early as 1957, the radiation

doses around “hot cells” used to store large quantities of radioactive materials were

reported to be high due to photon scattering (Rindi and Thomas, 1975). Both of these

phenomena were referred to as skyshine radiation. These observations led to a large

number of empirical and theoretical studies of radiation propagation and skyshine

radiation.

Several methods for calculating the expected skyshine radiation from x-ray and

gamma radiation facilities have been developed and are described by the National

Council on Radiation Protection and Measurements (NCRP 2005). These methods

include an equation for dose rate at a location based on the dose rate at isocenter, the

solid angle of the beam, and geometric considerations. McGinley (1993) measured

skyshine radiation levels in the vicinity of a medical accelerator producing 18 MV

1

photons. His results showed a discrepancy with the values calculated using NCRP (2005,

1977) methods.

The most common photon energies for medical accelerators are 6 MV and 10

MV. Neutron production is not a concern at these energies, but photon skyshine can

contribute a radiation dose to employees or members of the public. Even when all

radiation levels in the vicinity of the accelerator are below regulatory limits, the ALARA

principle requires that the radiation exposures be kept as low as reasonably achievable.

This requires knowledge of the radiation levels present.

The purpose of this work is to examine the skyshine radiation levels on the roof

and at ground level in the vicinity of a radiation therapy accelerator facility which

produces 6 MV and 10 MV photons. The radiation exposure rates were measured using a

pressurized ion chamber with an eight liter volume and the results were compared to the

values calculated using the equation in NCRP 151 (2005). This research shows that the

model in NCRP 151 does not fit the observed values of skyshine radiation.

2

Materials and Methods

The Accelerator Facility

A Varian Trilogy Accelerator (Varian Trilogy, Varian Medical Systems, Palo

Alto, CA 94304) was commissioned at the Colorado State University Veterinary Medical

Center in autumn 2007. The walls of the Edward L. Gillette Radiation Oncology Suite

are constructed of 1.5 m of concrete in the primary beam (Fig. 1). This provides

sufficient shielding from direct radiation. However, as is the case at many accelerator

facilities, the roof has minimal shielding and radiation escapes vertically. This radiation

is scattered in the atmosphere resulting in measurable levels of skyshine on the roof and

at ground level around the facility. N

Ground Locations Outside Facility

Scale = 1 meter

Control Room

A

= Axis of Gantry Rotation

Fig. 1. Plan view of the radiation therapy suite. The ground locations to the west are indicated by the arrow. Location A on the plan indicates where the constancy measurements were taken. The axis of rotation of the accelerator gantry is in the north-south direction at a height of 1.3 m above the floor.

3

The locations selected for skyshine measurements were at ground level along a

line to the west of the isocenter (Fig. 1) and at roof level, both outside directly south of

the isocenter and inside along a corridor (Fig. 2).

Outside Roof Locations

Inside 2nd Floor C orridor

Isocenter Below

Chain Link Fence

Scale = 1 meter

N

Fig. 2. Plan view of the roof of the radiation therapy suite. The locations for measurements at roof level are indicated by the arrows. Measurements were taken outside directly south of the isocenter location beginning at the second fence. Measurements were also taken inside along the corridor as shown. The Varian Trilogy is a linear accelerator for radiation oncology therapy. The

dose rate is calibrated at dmax in a water phantom with a source to surface distance (SSD)

of 100 cm. For 6 MV photons, dmax is at a depth of 1.5 cm and for 10 MV photons, dmax

is at a depth of 2.5 cm. The Varian Trilogy is capable of delivering 6 MV photons at

dose rates up to 1000 cGy min-1 and 10 MV photons at dose rates up to 600 cGy min-1

For the purposes of this study, the Trilogy accelerator was operated at a dose rate of 600

cGy min-1 during all measurements. The gantry rotates about the isocenter, a point 100

cm from the accelerator target (Fig 3). Data were obtained for vertical gantry (180°) for

all measurement locations. Data were also obtained with gantry angle 205° for the

ground level measurements and 155° for the corridor measurements (Fig. 3 and Fig. 4).

4

The collimators can be adjusted to dimensions from 0.5 cm to 40 cm measured at

isocenter. For this project, the collimator was closed to 0.5 cm × 0.5 cm beam to measure

leakage radiation. For skyshine measurements, a field size of 10 cm × 10 cm at isocenter

was used because it is the most common for traditional treatment plans. Measurements

were also taken for a 40 cm × 40 cm field because it is the largest field size available with

this accelerator. Gantry angle 0°

Isocenter

Gantry 155º (25° the east) Gantry angle 18

Beam Vertical

Gantry 205° (25to west)

0 °

Gantry 90° Gantry 270°

Fig 3. The gantry of the accelerator rotates about the isocenter. The angles are defined such that 0° is at the top with the beam directed downward. The angles increase in the clockwise direction as viewed facing the gantry.

Figure 4. The Varian Trilogy radiation therapy accelerator set to gantry angles of 180° and 205° for skyshine measurements.

5

Radiation Detection Instrumentation

The skyshine radiation measurements were made using a Reuter-Stokes Gamma

Radiation Sensor (RSS-120-100mR, Reuter-Stokes, Inc, 8499 Darrow Rd., Twinsburg,

Ohio 44087). The calibration certificate is in Appendix A. The RSS-120 is a spherical

ion chamber with an eight liter volume containing 25 atmospheres of argon. The unit

includes an auto-ranging electrometer with a low range of zero to 500 μR h-1 and a high

range from 0.5 to 100 mR h-1. A Fluke model 25 digital multimeter (Fluke Corporation,

6920 Seaway Blvd., Everett, WA 98206) was used to measure the output voltage. An

internal 300 V battery provides the sensor bias voltage and an external 12 V rechargeable

battery pack supplies DC power to the electrometer. The RSS-120 pressurized ion

chamber (PIC) and the case containing the readout multimeter and the 12 V battery were

placed on a plastic cart with a horizontal surface 0.75 meter above the ground (Fig. 5) for

all measurements.

Figure 5. The RSS-120 PIC and the case containing the 12 V battery pack and a Fluke digital multimeter for voltage readout were placed on a plastic cart for all measurements.

6

To measure and document instrument drift for the RSS-120, constancy

measurements were taken before and after each set of skyshine data. The PIC on the cart

was placed at position A as indicated in Fig. 1. Measurements were made with the gantry

facing west (270°) and the collimator closed (0.5 cm × 0.5 cm) for 6 MV and 10 MV

photons. These conditions yielded low range measurements from the electrometer.

Measurements were then made with the gantry facing east (90°) and collimator wide

open (40 cm × 40 cm) for 6 MV and 10 MV photons to test the high range.

For each location measurements were taken with the accelerator off to obtain

background levels, Measurements were also taken with the accelerator on and the

collimator closed to obtain leakage radiation measurements for each energy. For each

measurement of background, leakage, constancy or skyshine, five data points were

collected and averaged. The voltage readings were converted to exposure rates using the

calibration factor. The net skyshine radiation at each location and energy was determined

by subtracting the average background radiation and the average leakage, for that

location and energy, from the exposure rate measured.

In order to determine the transmission through the roof, the exposure rate in the

beam was measured at isocenter and in the center of the beam 0.90 meter above the roof.

Two instruments were used for these measurements to provide redundancy and validate

the results. The exposure rate was measured directly using a RadCal Model 2025

Radiation Monitor (Model 2025 Monitor with Model 20X5-3 Electrometer/Ion Chamber,

RadCal Corporation 426 West Duarte Road, Monrovia, CA 91016). The 3 cm3 ion

chamber was used with a buildup cap equivalent to 0.5 cm of water. In addition, a 0.6

cm3 Farmer chamber (PTW Model N30013 Farmer Chamber, PTW-Freiburg, Lörracher

7

Strasse 7, 79115 Freiburg, Germany) with an aluminum buildup cap equivalent to 2.5 cm

of water was used. The collected charge was displayed with a Keithley advanced therapy

dosimeter (Model 35040, Keithley Instruments, Inc., 28775 Aurora Road, Cleveland,

Ohio 44139). The exposure rate was determined by multiplying the charge (in nC) by the

calibration factor (Appendix C).

The distance from the floor of the therapy suite to the roof is 4.88 m and the

distance from the floor to the isocenter is 1.31 m. The RadCal chamber and the PTW

chamber were mounted on a stand 0.9 m above the level of the roof. When the gantry is

at 180°, the target to chamber distance is 5.47 m. The roof shielding transmission factor

for photons, Bxs, is calculated using equation 1:

iso

2tcchamber

xs XdX

B&

&= (1)

where:

is the exposure rate at the chamber location on the roof; chamberX&

is the exposure rate at the isocenter (100 cm from the target); isoX&

is the distance from the target to the chamber above the roof. tcd

Calculation of Expected Radiation Levels

The expected skyshine radiation levels at ground locations outside the accelerator facility

(Fig. 6) are calculated using the methods of McGinley (2002) as described in NCRP 151

(2005).

8

Fig 6. A simplified schematic showing the isocenter and the geometric variables used in equation 1 for photon skyshine calculation. (NCRP 2005).

Target

+ Isocenter

Ω

di

2 m

ds

Detector

The dose equivalent rate at a horizontal distance from the isocenter of the beam is

given by equation 2 (NCRP, 2005):

(2) ( )( )2

si

1.3xs0

7 105.2 dd

BDH

Ω×=

&&

where:

H& is the dose equivalent rate in nG h-1;

is the absorbed dose output rate at one meter from the source in Gy h-1; 0D&

Bxs is the roof shielding transmission factor for photons;

Ω is the solid angle of the beam in steradians;

di is the vertical distance from the target to a point two meters above the roof;

ds is the horizontal distance of the location of interest from the isocenter.

9

Results

Roof Transmission

The Varian Trilogy was set to deliver a dose rate of 600 cGy min-1 at dmax for an

SSD of 100 cm. Measurements of exposure rate were taken at isocenter and at a location

0.9 m above the roof. Data were obtained for 6 MV and 10 MV photons at a gantry angle

of 180° for 10 cm × 10 cm and 40 cm × 40 cm field sizes. The roof shielding

transmission factors, Bxs, to be used in later calculations are highlighted in Table 1. The

data for the 10 cm × 10 cm field size was used to calculate the transmission factors

because the 40 cm × 40 cm field size has significantly more scatter which increases the

reading at the chamber location. There is a discrepancy in the exposure rate values due to

insufficient buildup for the RadCal 20X5-3 in the beam at isocenter and a 5% uncertainty

in calibration for both probes.

Results from RadCal 2025 with Model 20X5-3 Probe including Cobalt-60 buildup cap Exposure Rate R min-1 Exposure Rate C kg¹ s¹ Isocenter Roof Isocenter Roof Ratio Bxs 6 MV 10x10 594.87 0.514 0.153 1.33E-04 0.0009 0.027 6 MV 40x40 655.08 1.098 0.169 2.83E-04 0.0017 10 MV 10x10 517.81 0.917 0.134 2.37E-04 0.0018 0.055 10 MV 40x40 611.73 1.881 0.158 4.85E-04 0.0031 Results from PTW Model N30013 Farmer Chamber with Aluminum Buildup Cap Exposure Rate R min-1 Exposure Rate C kg¹ s¹ Isocenter Roof Isocenter Roof Ratio Bxs 6 MV 10x10 639.37 0.564 0.165 1.46E-04 0.0009 0.027 6 MV 40x40 673.01 1.167 0.174 3.01E-04 0.0017 10 MV 10x10 684.04 1.151 0.176 2.97E-04 0.0017 0.052 10 MV 40x40 728.09 2.005 0.188 5.17E-04 0.0028

Table 1. Exposure rates and roof transmission factors calculated based on data obtained with the RadCal 2025 and with the PTW N30013 (Appendix C).

10

RSS-120 PIC Calibration and Constancy

The RSS-120 PIC was calibrated by K&S Associates (1926 Elm Tree Drive,

Nashville, TN 37210, 6 May 2008). The calibration factor for the low range (exposure

rates zero to 500 μR h-1) is 0.554 μR hr-1 mV-1. The calibration factor for the high range

(exposure rates 0.5 to 100 mR h-1) is 10.93 μR hr-1 mV-1. The accuracy of the calibration

factor is plus or minus 10% (Appendix A).

Measurements were taken using the RSS-120 PIC on six different days. To verify

that the instrument response was consistent over time, constancy measurements were

taken for background and high and low range conditions (Appendix B). The

measurements of background radiation had a coefficient of variation (CV) of 0.0406.

The CV for the 6 MV low range was 0.0335 and the CV for the 10 MV low range was

0.0329. For the 6 MV high range, the CV was 0.049. The measurements taken for 10

MV high range condition had a CV of 0.137. The exposure rate for the high range

condition (gantry 90°, collimator 40 cm × 40 cm) was at the upper end of the operating

range of the RSS-120 and the voltage output was dependent on the electrometer bias

voltage (Fig. 9). None of the PIC readings for the skyshine data were above 4 V high

range, which is well within the operating range of the RSS-120.

11

PIC Constancy

0

2

4

6

8

10

12

10 11 12 13Battery Voltage (with load)

PIC

Out

put (

V)

6 MV LowRange10 MV LowRange6 MV HighRange10 MV HighRangeBackground

Fig. 9. Average output voltages for the RSS-120 PIC during constancy measurements for the conditions indicated previously. The 10 MV high range output was sensitive to the voltage of the 12 V battery that provided the electrometer bias.

Skyshine Radiation at Ground Level Locations

The skyshine radiation was measured for locations beginning at the outside wall

of the radiation therapy suite and extending 50 meters to the west (Fig. 1). The exposure

rate data was placed in Appendix E. For a gantry angle of 180°, the location of the peak

is 10.9 m from the isocenter. The measured radiation levels at the peak for both photon

energies and field sizes are given in Table 2.

To normalize the skyshine exposure rate to a quantity that can be measured at

most medical accelerator facilities, the net skyshine exposure rate at a given location was

divided by the exposure rate for radiation exiting the roof above the isocenter. The ratio

was calculated for both energies and both field sizes (Fig. 10).

12

Exposure Rates in mR h-1

40 cm × 40 cm field 10 cm × 10 cm field

Gantry Location of Peak 6 MV 10 MV 6 MV 10 MV

180° 10.9 m 1.88 2.32 0.115 0.144

205° 10.0 m 1.99 2.69 0.146 0.197

Table 2. Skyshine exposure rates (mR h-1) measured at the location of the peak for both gantry angles, both energies and both field sizes. The transmission of photons increases with photon energy, but higher energy

photons scatter in the forward direction. The combination of these factors results in

higher net exposure rates for the 10 MV photons, but a lower ratio between the skyshine

radiation and the radiation leaving the roof above the isocenter (Fig. 10 a and b). The 40

cm × 40 cm field size has sixteen times the area of a 10 cm x 10 cm field size and results

in exposure rates sixteen times as high, as expected. The size of the error bars in Fig. 10

are calculated using propagation of errors, which include the 10% uncertainty in the RSS-

120 PIC calibration factor and the variation in background radiation, leakage and PIC

measurements.

For a gantry angle of 205° the peak exposure rate is greater than for a gantry angle

of 180°, and occurs closer to the wall. The transmission factor and exposure rate exiting

the roof were not determined for a 205° gantry angle, so ratios were not calculated.

13

Ratio of Skyshine Radiation to the Exposure Rate Exiting the Roof

for a 40 x 40 Field with 180° Gantry

0.00E+00

5.00E-06

1.00E-05

1.50E-05

2.00E-05

2.50E-05

3.00E-05

0 10 20 30 40 5Distance from Isocenter (m)

Rat

io

0

6 MV

10 MV

(a)

Ratio of Skyshine Radiation to the Exposure Rate Exiting the Roof

for 10 x 10 Field with 180° Gantry

0.00E+00

5.00E-07

1.00E-06

1.50E-06

2.00E-06

2.50E-06

3.00E-06

3.50E-06

4.00E-06


Rat

io

0

6 MV

10 MV

(b) Fig. 10. The net exposure rate for skyshine radiation at a location was divided by the exposure rate measured 0.9 m above the roof above isocenter. The gantry angle was 180° and the field sizes were (a) 40 cm × 40 cm and (b) 10 cm x 10 cm.

14

The expected skyshine radiation for ground and roof locations was calculated in

Appendix D using equation (2). The calculated and measured skyshine dose rates for

ground locations are compared in Fig. 11.

Skyshine Dose Rates at Ground Locations

For 40 x 40 Field Size

0

5

10

15

20

25

30

35

0 10 20 30 40 50Distance From Isocenter (m)

Net

Dos

e R

ate

(μG

y/hr

)

6 MV Calculated

6 MV Measured

10 MV Calculated

10 MV Measured

Skyshine Dose Rates at Ground Locations For 10 x 10 Field Size

0.0

0.5

1.0

1.5


Net

Dos

e R

ate

(μG

y/hr

)

6 MV Calculated

6 MV Measured

10 MV Calculated

10 MV Measured

(a)

(b) Fig. 11. Skyshine dose rates measured and calculated using equation 2 for (a) 40 cm × 40 cm and (b) 10 cm × 10 cm field sizes.

15

Skyshine Radiation Levels for Outside Locations at Roof Level The skyshine radiation was measured on the roof beginning at the second fence

(Fig. 2) and extending 50 meters south. The exposure rate data for outside roof locations

was placed in Appendix F. Only a vertical gantry angle (180°) was used for

measurements made along the line south of the isocenter. As shown in Fig. 12, for the

roof locations there is no peak in the skyshine radiation curve. The exposure rate

decreased with distance for all roof locations. The skyshine exposure rate was

normalized to the radiation exiting the roof above isocenter. The ratio of skyshine to

beam exiting the roof was calculated for both energies and both field sizes (Fig. 12).

Measurements were not obtained for locations inside the second fence (Fig. 2).

The plastic cart on which the PIC was placed for all measurements did not fit between the

inner and middle fences, and the values obtained were very different when the PIC was

set on the roof compared to on the cart. For example, at the location just outside the

second fence, the PIC sitting on the cart produced readings more than a factor of 3 higher

than when the PIC was sitting directly on the roof. At the location outside the outer

fence, the cart level readings were more than 68% higher than the roof level readings for

the same beam conditions.

Since the shape of the exposure rate curve for outdoor locations on the roof is

similar to the shape of the graph generated using equation (2), a comparison was made

between measured skyshine radiation and the calculated values (Appendix D).

The calculation of skyshine radiation using the same value of di as was used for the

ground locations (di = 6.57) resulted in estimates of skyshine that were significantly

lower than the observed radiation dose rates. (Fig. D.1). The di is the distance from the

16

target to a point 2 m above the roof, so the calculations were repeated with a di value of

2.0 m resulting in a better fit to the data (Fig. D.2). However, the calculated dose rate is

still an underestimation, and more work needs to be done to develop a model that fits the

data for skyshine radiation at roof level.

17

Ratio of Skyshine Radiationto the Exposure Rate Exiting the Roof

for a 40 x 40 Field with 180°Gantry

1.00E-06

1.00E-05

1.00E-04

1.00E-03

0 10 20 30 40 50

Distance from Isocenter (m)

Rat

io

6 MV

10 MV

(a)

Ratio of Skyshine Radiation

to the Exposure Rate Exiting the Roof for a 10 x 10 Field with 180° Gantry

1.00E-07

1.00E-06

1.00E-05

1.00E-04


Rat

io

6 MV10 MV

(b) Fig. 12. Ratio of skyshine radiation at a location on the roof to the exposure rate exiting the roof. The net exposure rate for skyshine radiation at a location was divided by the exposure rate measured 0.9 m above the roof above isocenter. The field sizes were (a) 40 cm × 40 cm and (b) 10 cm × 10 cm.

18

Skyshine Radiation Levels at Second Floor Corridor Locations

The skyshine dose rates were determined for locations beginning at the north wall

and extending 40 meters down the second floor corridor (Fig. 2). The net dose rates in

the corridor are no more than 6% of the values at the same distance outside.

Skyshine Dose Rates in Second Floor Corridor

40 x 40 Field Size

00.5

11.5

22.5

33.5

4

10 15 20 25 30 35 40 45

Distance from Isocenter (m)

Net

Dos

e R

ate

(μG

y/h)

10 MV G180

10 MV G 155

6 MV G 180

6 MV G 155

Background

(a) Skyshine Dose Rates in Second Floor Corridor

10 x 10 Field Size

0.1

0.15

0.2

0.25

0.3

0.35

0.4

10 15 20 25 30 35 40 45Distance from Isocenter (m)

Net

Dos

e R

ate

(μG

y/h)

10 MV G180

10 MV G1556 MV G180

6 MV G155

Background

(b) Fig. 14. The skyshine dose rates measured in the second floor corridor for 6 MV and 10 MV photons with gantry angles of 180° and 155°. The field sizes are (a) 40 cm × 40 cm and (b) 10 cm × 10 cm.

19

Discussion

The skyshine radiation observed at a location depends on the energy of the

photons, the field size and the gantry angle. As expected, the exposure rate is higher

when the gantry is pointing towards the location of the detector than when it is vertical.

At ground locations, the exposure rate increased as the detector was moved away

from the wall until a point between ten and eleven meters from the wall, then the

exposure rate decreased with r2 (Appendix E). The wall of the radiation therapy suite

shields locations near the wall from the scattered radiation. An unexpected result is that

the location of highest exposure rate for ground locations is closer to the wall when the

gantry is not vertical (Fig 15).

+ Isocenter

Gantry 180°

Gantry 205°

ds

Fig. 15. Skyshine radiation exposure rates at ground level locations have a peak at a location that is closer to the wall when the gantry is tilted towards the detector.

20

A chain link fence encloses an area extending 15.5 m west from the wall of the

radiation therapy suite. An unanticipated decline in readings was observed immediately

beyond the fence (Fig. 16).

Skysine Exposure Rates - Ground Locations

Gantry 205, Field 40 x 40

0

500

1000

1500

2000

2500

3000

0 10 20 30 40 50

Distance from Isocenter (meters)

Expo

sure

Rat

e (u

R/h

r)

6 MV

10 MV

Background

Fig. 16. The exposure rate immediately outside the chain link fence was noticeably reduced. The graphs of exposure rates for other beam conditions (Appendix E) show the same anomaly. McGinley (2002) obtained data for skyshine radiation at ground level outside a

facility with an 18 MV accelerator. The units have been changed and the results were

graphed to allow comparison with the results from this study (Fig. 17). In both cases, the

field size at isocenter was 40 cm × 40 cm and the gantry was vertical. McGinley used a

dose rate of 400 cGy/min at isocenter and a roof transmission of 1.0 (no attenuation). In

this study, the dose rate was 600 cGy/min at dmax and the roof transmission was 0.027 for

6 MV photons and 0.055 for 10 MV photons. In spite of these differences, the shapes of

the curves are similar. It is clear from these graphs that the equation (2) from NCRP

21

(2005) does not fit the data obtained at ground level locations outside an accelerator

facility.

Skyshine Dose Rates 40 x 40 Field Size, 10 MV

0

5

10

15

20

25

30

0 10 20 30 40Distance From Isocenter (m)

Net

Dos

e R

ate

(μG

y/h)

50

Measured

Calculated

(a)

McGinley (1993) Skyshine Dose Rates 40 x 40 Field, 18 MV

020406080

100120140160180200


Phot

on R

ate

(μG

y/h)

0

Measured

Calculated

(b) Fig. 17. The calculated and observed skyshine dose rates due to medical accelerators. The field sizes are 40 cm × 40 cm. (a) Skyshine dose rates observed in this study due to 10 MV photons. (b) Skyshine from an 18 MV accelerator (McGinley, 1993).

22

The author proposes that medical accelerator facilities with 6 MV or 10 MV

photon beams measure the exposure rate in the beam at a point 0.9 m above the roof and

multiply by the values in Fig. 10 to determine the expected skyshine radiation exposure

rates at ground level locations.

On the roof, there is no evidence of a peak in the exposure rate curve. This is

reasonable since there is no vertical wall to shield roof locations from the skyshine

radiation. The shape of the curve obtained using the equation in NCRP 151 (NCRP,

2005) is similar to the data obtained outside on the roof. Nevertheless, more data is

needed from facilities with different energies and geometries to determine an appropriate

model for skyshine radiation at roof locations. The author proposes that medical

accelerator facilities with 6 MV or 10 MV photon beams measure the exposure rate in the

beam at a point 0.9 m above the roof and multiply by the values in Fig. 12 to determine

the expected skyshine radiation exposure rates at roof level locations.

In the 2nd floor corridor, the skyshine radiation is reduced to less than 6% of the

outside values at the same distance from the isocenter (with a gantry angle of 155°).

When the gantry is vertical (180°), the dose rates in the hallway are approximately 2% of

the rates outside at the same distance from the isocenter The curves are not as smooth as

those obtained outside because the exposure rates are the same order of magnitude as the

background radiation with similar variability. In addition, differences in shielding

material within the roof (duct work, etc.) may cause the attenuation of the skyshine to

vary with location.

It was also observed that the exposure rate in the corridor dropped by up to 50%

when people walked by. It was surprising that the radiation was attenuated to such a

23

large degree by a person in the hallway. Measurements were repeated when this

occurred, but it is clear that the measurements made in the hallway were sensitive to the

conditions at the PIC location and between the PIC and the source.

It is important to put skyshine radiation in perspective. The relative importance of

primary beam radiation, leakage, scatter and skyshine depend on the treatment protocols

in use at the accelerator facility and the shielding in the walls and the roof. At a ground

level location 10.9 m from the isocenter at the CSU VMC accelerator facility, the

skyshine radiation is a maximum. For field sizes of 10 cm × 10 cm, if we assume the use

factor is the same for the roof and the wall (i.e. let U=0.25), the weekly dose due to

skyshine at that location is 5% of the dose due to the primary beam for 6 MV photons and

1% for 10 MV photons (Appendix H). In comparison, the dose from scatter is less than

4% and the dose from leakage is less than 0.1% of the dose due to the primary beam.

Scatter and leakage radiation are taken into consideration when designing the shielding

for a radiation therapy facility. Skyshine radiation should also be considered.

24

Conclusion

The dose rates due to skyshine radiation depend on the energy, the field size, the

gantry angle and the dose rate in the primary beam. They also are sensitive to the

conditions at the location of the measurement.

The equation used to model skyshine in NCRP publications (2005, 1977) is not a

good fit to the observed dose rates at ground level or on the roof outside a medical

accelerator facility. At ground level locations, there is a region in which the skyshine is

shielded by the wall. The dose rate increases to a peak location approximately 10 m from

the wall and then decreases with r2. For locations on the roof of the facility, there is no

region of shielding and thus no peak in the curve. The shape of the curve produced by

equation (2) is similar to the observed dose rates at roof level, but the parameters need to

be adjusted to obtain a good fit to the data.

The author recommends that a more accurate method for estimating skyshine

radiation in the vicinity of a medical accelerator facility is to measure the exposure rate of

the radiation exiting the roof of the facility and to scale the results using Fig. 10 for

ground level locations and Fig. 12 for locations on the roof.

Additional data is needed from facilities operating at different energies and with

different geometries in order to accurately estimate the skyshine radiation in the vicinity

of current and future medical accelerator facilities.

25

References

McGinley PH. Radiation skyshine produced by an 18 MeV medical accelerator. Radiat Prot Manage 10, 59-64; 1993. McGinley PH. Shielding techniques for radiation oncology facilities. 2nd ed. Madison, WI: Medical Physics Publishing; 2002: 101-106. National Council on Radiation Protection and Measurements. Structural shielding design and evaluation for megavoltage x- and gamma-ray radiotherapy facilities. Bethesda, MD: National Council on Radiation Protection and Measurements; NCRP Report No. 151; 2005: 84-88. National Council on Radiation Protection and Measurements. Radiation protection design guidelines for 0.1-100 MeV particle accelerator facilities. Washington, DC : National Council on Radiation Protection and Measurements; NCRP Report No. 51; 1977. Rindi A, Thomas RH. Skyshine – a paper tiger? Particle Accelerators 7: 23-39; 1975. Thomas RH. The history and future of accelerator radiological protection. Radiat Prot Dosimetry 96:441-457; 2001. Varian Medical Systems 3100 Hansen Way, Palo Alto, CA. [online] http://www.varian.com/us/oncology/radiation_oncology/trilogy/. Accessed 22 April 2008.

26

Appendix A – Calibration

27

Figure A.1. The calibration certificate for the RSS-120 PIC.

28

RSS-120 Low Range Calibration

y = 0.0554x - 2.5165

050

100150200250300350400450

0 2000 4000 6000 8000PIC Reading (mV)

Expo

sure

Rat

e (u

R/h

)

Figure A.2. The calibration curve for the low range of the RSS-120 PIC. The calibration factor is 0.0554 μR h-1 mV-1 (18.047 mV μR-1 h). The coefficient of determination, R2, is equal to 1.0.

RSS-120 High Range Calibration y = 10.947x - 31.324

0

1000020000

30000

4000050000

60000

7000080000

90000

0 1000 2000 3000 4000 5000 6000 7000 8000

PIC Reading (mV)

Expo

sure

Rat

e (u

R/h

)

Figure A.3. The calibration curve for the high range of the RSS-120 PIC. The calibration factor is 10.47 μR h-1 mV-1 (0.0913 mV μR-1 h). The coefficient of determination, R2, is equal to 1.0.

29

Appendix B – RSS-120 PIC Constancy

30

Table B.1. PIC Constancy Raw Data

Date: 3/5/2008Background: 0.317 0.326 0.340 0.331 0.332 Battery No Load 11.62

0.334 0.322 0.327 Battery w/ Load 11.50Constancy readings taken with gantry facing west (270°) and collimator closed (0.5x0.5)

Reading 1 Reading 2 Reading 3 Reading 4 Reading 56 MV 0.452 0.432 0.435 0.433 0.45910 MV 1.331 1.389 1.312 1.377 1.426

Constancy readings taken with gantry facing east (90°) and collimator wide open (40x40)Reading 1 Reading 2 Reading 3 Reading 4 Reading 5

6 MV 2.105 2.097 2.100 2.117 2.108 High Range10 MV 7.870 7.880 7.870 7.880 7.880 High Range

Date: 3/12/2008Background: 0.336 0.329 0.350 0.355 0.356 Battery No Load 11.26Background: 0.334 0.337 0.348 0.352 0.331 Battery w/ Load 9.82

Constancy readings taken with gantry facing west (270°) and collimator closed (0.5x0.5)Reading 1 Reading 2 Reading 3 Reading 4 Reading 5

6 MV 0.449 0.471 0.462 0.455 0.44310 MV 1.365 1.415 1.460 1.456 1.423



Date: 3/27/2008 3:00 PMBackground: 0.368 0.351 0.343 0.339 0.348 Battery No Load 12.25Background: 0.340 0.344 0.335 0.350 0.345 Battery w/ Load 11.83


6 MV 0.462 0.459 0.468 0.464 0.47310 MV 1.445 1.426 1.471 1.454 1.463





6 MV 0.462 0.459 0.468 0.464 0.47310 MV 1.445 1.426 1.471 1.454 1.463



31

Table B.1. Date:_ 3/27/2008 5:00 PMBackground: 0.340 0.343 0.352 0.352 0.343 Battery No Load 12.18Background: 0.351 0.406 0.355 0.353 0.348 Battery w/ Load 11.74


6 MV 0.474 0.491 0.486 0.475 0.45610 MV 1.507 1.489 1.491 1.509 1.510





6 MV 0.454 0.467 0.450 0.460 0.45410 MV 1.381 1.382 1.424 1.424 1.419



Date:_ 3/31/2008 5:00 PMBackground: 0.319 0.316 0.333 0.335 0.334 Battery No Load 12.73Background: 0.319 0.322 0.336 0.330 0.318 Battery w/ Load 12.36


6 MV 0.452 0.459 0.471 0.452 0.46610 MV 1.361 1.387 1.412 1.426 1.420





6 MV 0.463 0.476 0.492 0.469 0.46310 MV 1.450 1.435 1.460 1.476 1.456


6 MV 2.033 2.071 2.032 2.030 2.035 High Range

32

Table B.1. Date: 4/2/2008 5:00 PMBackground: 0.327 0.330 0.331 0.331 0.326 Battery No Load 12.93Background: Battery w/ Load 12.87


6 MV 0.473 0.474 0.464 0.458 0.47110 MV 1.482 1.543 1.500 1.534 1.499


6 MV 1.811 1.809 1.793 1.811 1.802 High Range

Date:_ 4/2/2008 8:00 PMBackground: 0.354 0.355 0.352 0.350 0.354 Battery No Load 12.92Background: Battery w/ Load 12.85


6 MV 0.445 0.480 0.476 0.496 0.48810 MV 1.440 1.468 1.426 1.446 1.442


6 MV 1.836 1.819 1.832 1.822 1.837 High Range

Date: 4/8/2008 4:15 PMBackground: 0.344 0.361 0.338 0.351 0.353 Battery No Load 13.01

Battery w/ Load 12.92


6 MV 0.456 0.428 0.433 0.458 0.46610 MV 1.438 1.404 1.427 1.434 1.403


6 MV 2.065 2.083 2.054 2.074 2.059 High Range

Date: 4/8/2008 5:40 AMBackground: 0.347 0.349 0.347 0.346 0.362 Battery No Load 12.96

Battery w/ Load 12.89


6 MV 0.450 0.450 0.469 0.439 0.45310 MV 1.411 1.394 1.428 1.410 1.401


6 MV 2.078 2.069 2.067 2.043 2.087 High Range

33

Table B.2. PIC Constancy Statistical Analysis Mean PIC

Reading (V)

Standard Deviation

Coefficient of

Variation 6 MV Low Range 0.461 0.015 0.03410 MV Low Range 1.436 0.047 0.0336 MV High Range 2.018 0.099 0.04910 MV High Range 9.033 1.238 0.137Background 0.341 0.014 0.041

34

Appendix C - Exposure Rate In Beam and Roof Shielding Transmission Factors

Date: 5/25/2008 Pressure: 635.5 mm Hg 84.52 kPa

Temperature: 22 °C

Instrument and Probe Instrument and ProbeRadCal 2025 Series Rate Mode Keithley Model 35040 1 min pulseRadCal Model 20X5-3 PTW Model N30013 with Cobalt Build-up cap Farmer Chamber w/Al Build-up cap

Calib. Factor = 5.689 R/nCRel to 22 °C, 101.3kPa Rel to 22 °C, 101.3 kPa

Location: At Isocenter Temp (°C) 23.4 At Isocenter Temp (°C) 23.4Temp Pres Factor = 1.2041995 Temp Pres Factor = 1.204199473

10x10 Field: Measured TP Corrected Measured (nC) TP Corr Exposure 6MV 494 594.87 R/min 93.33 639.37 R/min

10 MV 430 517.81 R/min 99.85 684.04 R/min40 x 40 Field:

6MV 544 655.08 R/min 98.24 673.01 R/min10 MV 508 611.73 R/min 106.28 728.09 R/min

Location: Roof Temp (°C) 14.5 Roof Temp (°C) 14.5Temp Pres Factor = 1.168041 Temp Pres Factor = 1.168040987

10x10 Field: Measured TP Corrected Measured (nC) TP Corr Exposure 6MV 0.440 0.514 R/min 0.085 0.56 R/min

10 MV 0.785 0.917 R/min 0.173 1.15 R/min40 x 40 Field:

6MV 0.940 1.098 R/min 0.176 1.17 R/min10 MV 1.610 1.881 R/min 0.302 2.00 R/min

Calculation of Roof Transmission Factor

Distance Target to roof = 4.57 metersDistance roof to chamber 0.98 metersTotal distance = 5.55 meters

Transmission Factor BxsRadCal 10x10 40x40 Farmer 10x10 40x40

6 MV 0.027 0.052 6 MV 0.027 0.05310 MV 0.055 0.095 10 MV 0.052 0.085

35

Appendix D - Calculation of Expected Skyshine Dose Rate Using NCRP 151 Method

Equation (2) is used to calculate the expected dose equivalent rate, H& , due to skyshine

(NCRP, 2005).

( )

( )2si

1.3xs0

7 105.2 dd

BDH

Ω×=

&& (2)

Roof Transmission Factors used for calculations (Bxs) 6MV: Bxs = 0.027 10 MV: Bxs = 0.052 Solid Angle of the Beam Ω=2π(1-cosθ)

r

θ

For 10 cm × 10 cm field: r = 5.64 cm (circle with area equivalent to 10 cm × 10 cm square) θ = arctan (5.64/100) = 3.23 degrees = 0.056 radians Ω = 0.010 steradians For 40 cm × 40 cm field: r = 22.6 cm (circle with area equivalent to 40 cm × 40 cm square) θ = arctan (22.6/100) = 12.72 degrees = 0.222 radians Ω = 0.154 steradians Dose Rate = =360 Gy/hr (600 cGy/minute) 0D&

di = 6.57 m (from target to 2 m above the roof) for ground locations di = 2.00 m (chosen for good fit to all roof data) for roof locations

36

Skyshine Dose Rates at Roof Locations For 40 x 40 Field Size

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40Distance from Isocenter (m)

Dos

e R

ate

(μG

y/h)

50

6 MV Measured

10 MV Measured

6MV Calculated

10 MV Calculated

(a)


02468

101214161820

0 10 20 30 40 50 6Distance from Isocenter (m)

Dos

e R

ate

(μG

y/h)

0

6 MV Measured

10 MV Measured

6 MV Calculated

10 MV Calculated

(b)

Figure D.1. Measured and calculated skyshine radiation at roof locations. Calculations were made using equation (2) with di = 6.57.

37


0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40Distance from Isocenter (m)

Dos

e R

ate

(μG

y/h)

50

6 MV Measured

10 MV Measured

6MV Calculated

10 MV Calculated

(a)


02468

101214161820

0 10 20 30 40 50 6Distance from Isocenter (m)

Dos

e R

ate

(μG

y/h)

0

6 MV Measured

10 MV Measured

6 MV Calculated

10 MV Calculated

(b)

Fig. D.2. Measured and calculated skyshine radiation at roof locations. Calculations were made using equation (2) with di = 2.0. The fit is better than with di = 6.57, but exposure rates are still underestimated.

38

Table D.1. Calculation of Expected Dose Rate Using NCRP 151 Method 6 MV 10 x 10 6 MV 40 x 40

Ground Roof Ground RoofDistance to Dose Rate Dose Rate Distance to Dose Rate Dose Rate

Isocenter (m) (μSv/hr) (μSv/hr) Isocenter (m) (μSv/hr) (μSv/hr)4.52 4.222 45.563 4.52 25.77 278.1135.44 2.921 31.523 5.44 17.83 192.4116.35 2.140 23.098 6.35 13.06 140.9867.27 1.635 17.649 7.27 9.98 107.7268.18 1.290 13.923 8.18 7.88 84.9859.10 1.044 11.264 9.10 6.37 68.752

10.01 0.862 9.299 10.01 5.26 56.76210.92 0.723 7.807 10.92 4.42 47.65511.84 0.616 6.648 11.84 3.76 40.57712.75 0.531 5.728 12.75 3.24 34.96613.67 0.462 4.987 13.67 2.82 30.44314.58 0.406 4.381 14.58 2.48 26.74415.50 0.360 3.880 15.50 2.19 23.68016.41 0.321 3.459 16.41 1.96 21.11518.24 0.259 2.800 18.24 1.58 17.09220.07 0.214 2.313 20.07 1.31 14.11924.64 0.142 1.534 24.64 0.87 9.36529.22 0.101 1.092 29.22 0.62 6.66335.31 0.069 0.747 35.31 0.42 4.56142.63 0.048 0.513 42.63 0.29 3.12949.95 0.035 0.373 49.95 0.21 2.280

10 MV 10 x 10 10 MV 40 x 40Ground Roof Ground Roof

Distance to Dose Rate Dose Rate Distance to Dose Rate Dose Rate Isocenter (m) (μSv/hr) (μSv/hr) Isocenter (m) (μSv/hr) (μSv/hr)

4.52 8.70 93.88 4.52 53.70 579.465.44 6.02 64.95 5.44 37.15 400.906.35 4.41 47.59 6.35 27.22 293.757.27 3.37 36.36 7.27 20.80 224.458.18 2.66 28.69 8.18 16.41 177.079.10 2.15 23.21 9.10 13.27 143.25

10.01 1.78 19.16 10.01 10.96 118.2710.92 1.49 16.09 10.92 9.20 99.2911.84 1.27 13.70 11.84 7.83 84.5412.75 1.09 11.80 12.75 6.75 72.8513.67 0.95 10.28 13.67 5.88 63.4314.58 0.84 9.03 14.58 5.16 55.7215.50 0.74 7.99 15.50 4.57 49.3416.41 0.66 7.13 16.41 4.08 43.9918.24 0.53 5.77 18.24 3.30 35.6120.07 0.44 4.77 20.07 2.73 29.4224.64 0.29 3.16 24.64 1.81 19.5129.22 0.21 2.25 29.22 1.29 13.8835.31 0.14 1.54 35.31 0.88 9.5042.63 0.10 1.06 42.63 0.60 6.5249.95 0.07 0.77 49.95 0.44 4.75

39

Appendix E – Skyshine Data and Analysis For Ground Locations

40

Table E.1. Raw Data for skyshine measurements – Ground Locations to West Date: 3/27/08 All Readings are in Volts.

Location: at wall Reading 1 Reading 2 Reading 3 Reading 4 Reading 5

Background Readings: 0.338 0.329 0.328 0.325 0.326Collimator Closed 6 MV 0.545 0.537 0.531 0.509 0.505Collimator Closed 10 MV 0.596 0.631 0.650 0.607 0.617

Gantry 180 degrees (vertical) Collimator 40x40 HIGH RANGE

6 MV Photons 0.070 0.071 0.070 0.071 0.07010 MV Photons 0.086 0.087 0.088 0.087 0.087

Collimator 10x106 MV Photons 1.349 1.349 1.337 1.341 1.343

10 MV Photons 1.544 1.617 1.617 1.608 1.601

Gantry 205 degrees (west) Collimator 40x40 HIGH RANGE



10 MV Photons 2.192 2.200 2.186 2.199 2.224

Location: 3 ft from wall Reading 1 Reading 2 Reading 3 Reading 4 Reading 5





10 MV Photons 2.037 2.059 2.027 2.055 2.035




10 MV Photons 2.789 2.805 2.790 2.821 2.847






10 MV Photons 2.347 2.348 2.346 2.366 2.379




10 MV Photons 3.235 3.250 3.260 3.280 3.260

41

Table E.1 Location: 9 ft from wall

Reading 1 Reading 2 Reading 3 Reading 4 Reading 5Background Readings: 0.321 0.322 0.319 0.301 0.303Collimator Closed 6 MV 0.426 0.455 0.444 0.432 0.413Collimator Closed 10 MV 0.509 0.509 0.537 0.535 0.534




10 MV Photons 2.682 2.656 2.657 2.673 2.668




10 MV Photons 3.670 3.660 3.670 3.660 3.680

Location: 12 ft from wallReading 1 Reading 2 Reading 3 Reading 4 Reading 5





10 MV Photons 2.890 2.883 2.895 2.890 2.857




10 MV Photons 3.940 3.930 3.940 3.930 3.920






10 MV Photons 3.056 3.043 3.107 3.082 3.058




10 MV Photons 4.040 4.040 4.060 4.070 4.040

42






10 MV Photons 3.097 3.111 3.131 3.127 3.130




10 MV Photons 4.080 4.110 4.100 4.120 4.130

Date: __4/2/08___________

Location: at wall Reading 1 Reading 2 Reading 3 Reading 4 Reading 5





10 MV Photons 1.677 1.669 1.691 1.667 1.699




10 MV Photons 2.222 2.224 2.243 2.233 2.224






10 MV Photons 2.667 2.689 2.680 2.690 2.688




10 MV Photons 3.590 3.620 3.610 3.640 3.660

43






10 MV Photons 3.073 3.079 3.084 3.083 3.110




10 MV Photons 4.040 4.050 4.040 4.060 4.070






10 MV Photons 3.113 3.100 3.125 3.141 3.133




10 MV Photons 4.030 4.020 4.050 4.060 4.000






10 MV Photons 3.134 3.097 3.082 3.094 3.096




10 MV Photons 3.990 3.970 3.990 3.980 3.930

44






10 MV Photons 3.050 3.062 3.061 3.069 3.059




10 MV Photons 3.790 3.820 3.830 3.840 3.830






10 MV Photons 2.978 2.940 3.010 3.022 3.040




10 MV Photons 3.700 3.710 3.710 3.740 3.720






10 MV Photons 2.945 2.952 2.975 2.949 2.952




10 MV Photons 3.600 3.590 3.610 3.620 3.650

45






10 MV Photons 2.756 2.757 2.743 2.748 2.766




10 MV Photons 3.370 3.380 3.390 3.400 3.390






10 MV Photons 2.737 2.731 2.709 2.740 2.753




10 MV Photons 3.290 3.310 3.320 3.310 3.300






10 MV Photons 2.682 2.671 2.678 2.654 2.675




10 MV Photons 3.151 3.148 3.152 3.162 3.142

46






10 MV Photons 2.520 2.520 2.526 2.514 2.549




10 MV Photons 2.985 2.994 2.985 2.987 2.994






10 MV Photons 2.195 2.224 2.224 2.242 2.226




10 MV Photons 2.504 2.495 2.501 2.501 2.511






10 MV Photons 1.926 1.934 1.942 1.934 1.955




10 MV Photons 2.184 2.177 2.187 2.198 2.188

47






10 MV Photons 1.639 1.660 1.669 1.687 1.668




10 MV Photons 1.861 1.848 1.857 1.840 1.854






10 MV Photons 1.398 1.388 1.358 1.382 1.370




10 MV Photons 1.535 1.530 1.537 1.566 1.547


Background Readings: 0.287 0.278 0.285 0.301 0.300Collimator Closed 6 MVCollimator Closed 10 MV




10 MV Photons 1.238 1.236 1.235 1.243 1.252




10 MV Photons 1.386 1.387 1.377 1.396 1.388

48

Table E.2. Analysis of skyshine measurements for ground locations to west. The low range calibration factor is 0.018147 V/(μR/h) and the high range calibration factor is 0.0000915 V/(μR/h).

Average Std Dev Avg Exposure Stdev NET Exp Stdev ExpLocation: at wall

Background 0.3292 0.0052 18.14 1.84Leakage 6 MV 0.5254 0.0176 28.95 3.05 12.41 3.67Leakage 10 MV 0.6202 0.0211 34.18 3.61 17.64 4.15

G180 Open 6 MV 0.0704 0.0005 769.40 77.17 740.45 77.29G180 Open 10 MV 0.0870 0.0007 950.82 95.40 916.64 95.51

G180 10x10 6 MV 1.3438 0.0052 74.05 7.41 45.10 8.52G180 10x10 10 MV 1.5974 0.0306 88.03 8.96 53.85 10.09

G205 Open 6 MV 0.0850 0.0007 928.96 93.22 900.01 93.31G205 Open 10 MV 0.1184 0.0005 1293.99 129.54 1259.81 129.62

G205 10x10 6 MV 1.6678 0.0090 91.90 9.20 62.95 10.12G205 10x10 10 MV 2.2002 0.0145 121.24 12.15 87.07 13.00

Average Std Dev Avg Exposure Stdev NET Exp Stdev Exp

Location: 3 ft from wall


G180 Open 6 MV 0.0960 0.0007 1049.18 105.20 1022.86 105.28G180 Open 10 MV 0.1190 0.0007 1300.55 130.28 1268.49 130.36

G180 10x10 6 MV 1.5924 0.0096 87.75 8.79 61.43 9.63G180 10x10 10 MV 2.0426 0.0137 112.56 11.28 80.50 12.09

G205 Open 6 MV 0.1130 0.0007 1234.97 123.74 1208.65 123.80G205 Open 10 MV 0.1544 0.0005 1687.43 168.85 1655.37 168.91

G205 10x10 6 MV 2.1298 0.0241 117.36 11.81 91.05 12.45G205 10x10 10 MV 2.8104 0.0243 154.87 15.54 122.81 16.14




G180 Open 6 MV 0.1188 0.0008 1298.36 130.16 1272.73 130.22G180 Open 10 MV 0.1472 0.0008 1608.74 161.13 1577.85 161.19

G180 10x10 6 MV 1.8964 0.0119 104.50 10.47 78.87 11.17G180 10x10 10 MV 2.3572 0.0147 129.89 13.01 99.00 13.69

G205 Open 6 MV 0.1368 0.0008 1495.08 149.79 1469.45 149.84G205 Open 10 MV 0.1864 0.0011 2037.16 204.10 2006.27 204.14

G205 10x10 6 MV 2.4380 0.0087 134.35 13.44 108.71 13.99G205 10x10 10 MV 3.2570 0.0164 179.48 17.97 148.59 18.47

49

Table E.2

Average Std Dev Avg Exposure Stdev NET Exp Stdev ExpLocation: 9 ft from wall


G180 Open 6 MV 0.1386 0.0005 1514.75 151.59 1490.84 151.64G180 Open 10 MV 0.1720 0.0007 1879.78 188.14 1850.86 188.18

G180 10x10 6 MV 2.1162 0.0005 116.61 11.66 92.70 12.28G180 10x10 10 MV 2.6672 0.0008 146.98 14.70 118.06 15.28

G205 Open 6 MV 0.1586 0.0005 1733.33 173.44 1709.42 173.48G205 Open 10 MV 0.2138 0.0008 2336.61 233.84 2307.69 233.88

G205 10x10 6 MV 2.7518 0.0115 151.64 15.18 127.72 15.66G205 10x10 10 MV 3.6680 0.0084 202.13 20.22 173.21 20.64




G180 Open 6 MV 0.1544 0.0005 1687.43 168.85 1663.66 168.89G180 Open 10 MV 0.1926 0.0011 2104.92 210.86 2075.48 210.90

G180 10x10 6 MV 2.3206 0.0111 127.88 12.80 104.11 13.35G180 10x10 10 MV 2.8830 0.0151 158.87 15.91 129.43 16.45

G205 Open 6 MV 0.1730 0.0007 1890.71 189.23 1866.94 189.27G205 Open 10 MV 0.2334 0.0005 2550.82 255.15 2521.38 255.19

G205 10x10 6 MV 2.9578 0.0089 162.99 16.31 139.22 16.74G205 10x10 10 MV 3.9320 0.0084 216.67 21.67 187.24 22.07




G180 Open 6 MV 0.1652 0.0008 1805.46 180.78 1782.75 180.82G180 Open 10 MV 0.2040 0.0010 2229.51 223.22 2200.53 223.26

G180 10x10 6 MV 2.4076 0.0094 132.67 13.28 109.96 13.79G180 10x10 10 MV 3.0692 0.0254 169.13 16.97 140.16 17.46

G205 Open 6 MV 0.1806 0.0011 1973.77 197.77 1951.06 197.80G205 Open 10 MV 0.2446 0.0005 2673.22 267.39 2644.25 267.42

G205 10x10 6 MV 3.0718 0.0161 169.27 16.95 146.56 17.35G205 10x10 10 MV 4.0500 0.0141 223.18 22.33 194.20 22.71

50

Table E.2



G180 Open 6 MV 0.1712 0.0008 1871.04 187.33 1849.13 187.36G180 Open 10 MV 0.2128 0.0008 2325.68 232.75 2297.90 232.78

G180 10x10 6 MV 2.4524 0.0092 135.14 13.52 113.23 14.01G180 10x10 10 MV 3.1192 0.0148 171.89 17.21 144.10 17.67

G205 Open 6 MV 0.1839 0.0014 2009.84 201.54 1987.93 201.57G205 Open 10 MV 0.2487 0.0015 2718.03 272.29 2690.25 272.32

G205 10x10 6 MV 3.0648 0.0144 168.89 16.91 146.98 17.30G205 10x10 10 MV 4.0800 0.0333 224.83 22.56 197.05 22.91

4/2/2008

Average Std Dev Avg Exposure Stdev NET Exp Stdev ExpLocation: at wall


G180 Open 6 MV 0.0734 0.0005 802.19 80.44 780.28 80.54G180 Open 10 MV 0.0904 0.0005 987.98 98.98 960.19 99.08

G180 10x10 6 MV 1.3720 0.0142 75.60 7.60 53.69 8.62G180 10x10 10 MV 1.6806 0.0140 92.61 9.29 64.83 10.32

G205 Open 6 MV 0.0880 0.0007 961.75 96.48 939.84 96.57G205 Open 10 MV 0.1196 0.0005 1307.10 130.85 1279.32 130.92

G205 10x10 6 MV 1.6792 0.0016 92.53 9.25 70.62 10.11G205 10x10 10 MV 2.2292 0.0088 122.84 12.29 95.06 13.09




G180 Open 6 MV 0.1390 0.0007 1519.13 152.11 1497.22 152.16G180 Open 10 MV 0.1726 0.0009 1886.34 188.89 1858.55 188.93

G180 10x10 6 MV 2.0922 0.0135 115.29 11.55 93.38 12.15G180 10x10 10 MV 2.6828 0.0097 147.84 14.79 120.05 15.36

G205 Open 6 MV 0.1582 0.0008 1728.96 173.14 1707.05 173.18G205 Open 10 MV 0.2140 0.0010 2338.80 234.13 2311.01 234.17

G205 10x10 6 MV 2.7274 0.0272 150.29 15.10 128.38 15.56G205 10x10 10 MV 3.6240 0.0270 199.70 20.03 171.92 20.45

51

Table E.2



G180 Open 6 MV 0.1716 0.0009 1875.41 187.80 1853.50 187.83G180 Open 10 MV 0.2120 0.0007 2316.94 231.82 2289.16 231.86

G180 10x10 6 MV 2.4580 0.0058 135.45 13.55 113.54 14.03G180 10x10 10 MV 3.0858 0.0142 170.04 17.02 142.26 17.47

G205 Open 6 MV 0.1828 0.0008 1997.81 199.99 1975.90 200.02G205 Open 10 MV 0.2476 0.0011 2706.01 270.89 2678.23 270.92

G205 10x10 6 MV 3.0390 0.0176 167.47 16.77 145.56 17.17G205 10x10 10 MV 4.0520 0.0130 223.29 22.34 195.50 22.68




G180 Open 6 MV 0.1744 0.0005 1906.01 190.70 1884.10 190.73G180 Open 10 MV 0.2150 0.0007 2349.73 235.10 2321.94 235.13

G180 10x10 6 MV 2.4920 0.0064 137.32 13.74 115.41 14.20G180 10x10 10 MV 3.1224 0.0162 172.06 17.23 144.28 17.68

G205 Open 6 MV 0.1836 0.0009 2006.56 200.89 1984.65 200.93G205 Open 10 MV 0.2466 0.0009 2695.08 269.69 2667.30 269.71

G205 10x10 6 MV 3.0320 0.0146 167.08 16.73 145.17 17.11G205 10x10 10 MV 4.0320 0.0239 222.19 22.26 194.40 22.61




G180 Open 6 MV 0.1740 0.0007 1901.64 190.32 1879.73 190.35G180 Open 10 MV 0.2154 0.0005 2354.10 235.49 2326.31 235.52

G180 10x10 6 MV 2.4890 0.0134 137.16 13.74 115.25 14.20G180 10x10 10 MV 3.1006 0.0196 170.86 17.12 143.08 17.54

G205 Open 6 MV 0.1800 0.0007 1967.21 196.87 1945.30 196.91G205 Open 10 MV 0.2428 0.0008 2653.55 265.51 2625.77 265.54

G205 10x10 6 MV 2.9974 0.0133 165.17 16.53 143.26 16.92G205 10x10 10 MV 3.9720 0.0249 218.88 21.93 191.09 22.26

52

Table E.2



G180 Open 6 MV 0.1730 0.0007 1890.71 189.23 1868.80 189.26G180 Open 10 MV 0.2130 0.0010 2327.87 233.04 2300.08 233.07

G180 10x10 6 MV 2.4434 0.0102 134.64 13.48 112.73 13.96G180 10x10 10 MV 3.0602 0.0068 168.63 16.87 140.85 17.28

G205 Open 6 MV 0.1756 0.0005 1919.13 192.01 1897.22 192.04G205 Open 10 MV 0.2360 0.0007 2579.23 258.04 2551.45 258.07

G205 10x10 6 MV 2.9216 0.0206 161.00 16.14 139.09 16.54G205 10x10 10 MV 3.8220 0.0192 210.61 21.09 182.83 21.42




G180 Open 6 MV 0.1694 0.0005 1851.37 185.23 1829.46 185.27G180 Open 10 MV 0.2084 0.0011 2277.60 228.10 2249.81 228.13

G180 10x10 6 MV 2.4260 0.0123 133.69 13.39 111.78 13.83G180 10x10 10 MV 2.9980 0.0395 165.21 16.66 137.42 17.07

G205 Open 6 MV 0.1706 0.0011 1864.48 186.86 1842.57 186.90G205 Open 10 MV 0.2292 0.0008 2504.92 250.66 2477.13 250.69

G205 10x10 6 MV 2.7976 0.0237 154.16 15.47 132.25 15.85G205 10x10 10 MV 3.7160 0.0152 204.77 20.49 176.99 20.83




G180 Open 6 MV 0.1656 0.0011 1809.84 181.41 1787.93 181.44G180 Open 10 MV 0.2040 0.0010 2229.51 223.22 2201.72 223.25

G180 10x10 6 MV 2.3694 0.0124 130.57 13.07 108.66 13.52G180 10x10 10 MV 2.9546 0.0118 162.81 16.29 135.03 16.70

G205 Open 6 MV 0.1650 0.0007 1803.28 180.49 1781.37 180.53G205 Open 10 MV 0.2228 0.0029 2434.97 245.62 2407.19 245.65

G205 10x10 6 MV 2.7148 0.0131 149.60 14.98 127.69 15.37G205 10x10 10 MV 3.6140 0.0230 199.15 19.96 171.37 20.29

53

Table E.2



G180 Open 6 MV 0.1550 0.0007 1693.99 169.58 1672.08 169.61G180 Open 10 MV 0.1916 0.0011 2093.99 209.77 2066.20 209.80

G180 10x10 6 MV 2.2312 0.0209 122.95 12.35 101.04 12.82G180 10x10 10 MV 2.7540 0.0089 151.76 15.18 123.98 15.61

G205 Open 6 MV 0.1530 0.0007 1672.13 167.39 1650.22 167.43G205 Open 10 MV 0.2060 0.0012 2251.37 225.53 2223.58 225.56

G205 10x10 6 MV 2.5590 0.0188 141.02 14.14 119.11 14.55G205 10x10 10 MV 3.3860 0.0114 186.59 18.67 158.80 19.02




G180 Open 6 MV 0.1546 0.0005 1689.62 169.07 1667.71 169.11G180 Open 10 MV 0.1916 0.0005 2093.99 209.48 2066.20 209.54

G180 10x10 6 MV 2.1854 0.0112 120.43 12.06 98.52 12.61G180 10x10 10 MV 2.7340 0.0161 150.66 15.09 122.87 15.88

G205 Open 6 MV 0.1514 0.0009 1654.64 165.75 1632.73 165.79G205 Open 10 MV 0.2044 0.0011 2233.88 223.74 2206.10 223.79

G205 10x10 6 MV 2.4968 0.0161 137.59 13.79 115.68 14.27G205 10x10 10 MV 3.3060 0.0114 182.18 18.23 154.39 18.89




G180 Open 6 MV 0.1512 0.0004 1652.46 165.32 1630.55 165.36G180 Open 10 MV 0.1850 0.0007 2021.86 202.33 1994.07 202.36

G180 10x10 6 MV 2.1470 0.0145 118.31 11.86 96.40 12.36G180 10x10 10 MV 2.6720 0.0108 147.24 14.74 119.46 15.15

G205 Open 6 MV 0.1434 0.0005 1567.21 156.84 1545.30 156.87G205 Open 10 MV 0.1924 0.0011 2102.73 210.64 2074.95 210.67

G205 10x10 6 MV 2.4142 0.0162 133.04 13.33 111.13 13.78G205 10x10 10 MV 3.1510 0.0073 173.64 17.37 145.85 17.72

54

Table E.2



G180 Open 6 MV 0.1432 0.0008 1565.03 156.77 1543.12 156.81G180 Open 10 MV 0.1762 0.0008 1925.68 192.79 1897.90 192.82

G180 10x10 6 MV 2.0782 0.0147 114.52 11.48 92.61 11.98G180 10x10 10 MV 2.5258 0.0136 139.19 13.94 111.40 14.38

G205 Open 6 MV 0.1348 0.0004 1473.22 147.40 1451.31 147.44G205 Open 10 MV 0.1806 0.0005 1973.77 197.47 1945.99 197.50

G205 10x10 6 MV 2.2792 0.0177 125.60 12.60 103.69 13.06G205 10x10 10 MV 2.9890 0.0046 164.71 16.47 136.93 16.85




G180 Open 6 MV 0.1216 0.0009 1328.96 133.26 1307.05 133.30G180 Open 10 MV 0.1494 0.0005 1632.79 163.39 1605.00 163.43

G180 10x10 6 MV 1.8108 0.0206 99.79 10.04 77.88 10.64G180 10x10 10 MV 2.2222 0.0170 122.46 12.28 94.67 12.77

G205 Open 6 MV 0.1102 0.0008 1204.37 120.78 1182.46 120.83G205 Open 10 MV 0.1484 0.0013 1621.86 162.85 1594.07 162.88

G205 10x10 6 MV 1.9494 0.0118 107.42 10.76 85.51 11.32G205 10x10 10 MV 2.5024 0.0058 137.90 13.79 110.11 14.23




G180 Open 6 MV 0.1054 0.0005 1151.91 115.35 1130.00 115.40G180 Open 10 MV 0.1294 0.0005 1414.21 141.55 1386.42 141.59

G180 10x10 6 MV 1.5728 0.0176 86.67 8.72 64.76 9.36G180 10x10 10 MV 1.9382 0.0110 106.81 10.70 79.02 11.25

G205 Open 6 MV 0.0946 0.0005 1033.88 103.56 1011.97 103.62G205 Open 10 MV 0.1270 0.0010 1387.98 139.23 1360.19 139.27

G205 10x10 6 MV 1.6962 0.0029 93.47 9.35 71.56 9.95G205 10x10 10 MV 2.1868 0.0076 120.50 12.06 92.72 12.55

55

Table E.2


Background 0.2620 0.0161 14.44 1.70Leakage 6 MV 0.3230 0.0079 17.80 1.83 1.26 2.75Leakage 10 MV 0.2798 0.0095 15.42 1.63 -1.12 2.61

G180 Open 6 MV 0.0894 0.0005 977.05 97.89 955.14 97.95G180 Open 10 MV 0.1088 0.0008 1189.07 119.26 1161.29 119.30

G180 10x10 6 MV 1.3774 0.0161 75.90 7.64 53.99 8.37G180 10x10 10 MV 1.6646 0.0174 91.73 9.22 63.94 9.80

G205 Open 6 MV 0.0796 0.0005 869.95 87.20 848.04 87.27G205 Open 10 MV 0.1064 0.0005 1162.84 116.44 1135.06 116.49

G205 10x10 6 MV 1.4562 0.0117 80.24 8.05 58.33 8.75G205 10x10 10 MV 1.8520 0.0082 102.06 10.22 74.27 10.74



Background 0.2798 0.0118 15.42 1.67Leakage 6 MV 0.2868 0.0092 15.80 1.66 -0.74 2.63Leakage 10 MV 0.3228 0.0058 17.79 1.81 1.25 2.73

G180 Open 6 MV 0.0710 0.0007 775.96 77.98 754.05 78.05G180 Open 10 MV 0.0872 0.0008 953.01 95.74 925.22 95.80

G180 10x10 6 MV 1.1564 0.0101 63.72 6.40 41.81 7.21G180 10x10 10 MV 1.3792 0.0156 76.00 7.65 48.22 8.37

G205 Open 6 MV 0.0626 0.0005 684.15 68.68 662.24 68.76G205 Open 10 MV 0.0844 0.0005 922.40 92.43 894.62 92.50

G205 10x10 6 MV 1.1956 0.0050 65.88 6.59 43.97 7.39G205 10x10 10 MV 1.5430 0.0143 85.03 8.54 57.24 9.20



Background 0.2902 0.0100 15.99 1.69

G180 Open 6 MV 0.0624 0.0005 681.97 68.46 660.06 68.49G180 Open 10 MV 0.0766 0.0005 837.16 83.93 809.37 83.95

G180 10x10 6 MV 1.0456 0.0062 57.62 5.77 35.71 6.12G180 10x10 10 MV 1.2408 0.0070 68.37 6.85 40.59 7.15

G205 Open 6 MV 0.0554 0.0005 605.46 60.84 583.55 60.88G205 Open 10 MV 0.0744 0.0005 813.11 81.53 785.33 81.56

G205 10x10 6 MV 1.0912 0.0075 60.13 6.03 38.22 6.36G205 10x10 10 MV 1.3868 0.0068 76.42 7.65 48.64 7.92

56

In Table E.2, “Average” and “Std Dev” refer to the PIC voltage measurements. The

“Avg Exposure” is the average exposure rate (in μR/h) calculated using the appropriate

calibration factor. The “Stdev” is the standard deviation of the average exposure rate

taking into account the variation in the data and the 10% uncertainty in the calibration

factor. “NET Exp” is obtained by subtracting net leakage and average background

exposure rates from the average exposure rate for the location. “Stdev Exp” takes into

account the propagation of error from the measurement, the calibration factors, the

leakage and the background.

57

Skyshine Exposure Rates - Ground Locations Gantry 180, Field 40 x 40

0

500

1000

1500

2000

2500

0 10 20 30 40 50Distance from isocenter (meters)

Expo

sure

Rat

e (u

R/h

r)6 MV

10 MV

Background

(a)

Skyshine Exposure Rates - Ground Locations

Gantry 205, Field 40 x 40

0

500

1000

1500

2000

2500

3000

0 10 20 30 40 50


Expo

sure

Rat

e (u

R/h

r) 6 MV

10 MV

Background

(b)

Fig. E.1. Skyshine exposure rates obtained at ground locations for 40 cm × 40 cm field size. Locations are ground level to the west of the accelerator facility. The gantry angles were (a) 180° and (b) 205°.

58


0

50

100

150

200

250

0 10 20 30 40Distance from Isocenter (meters)

Exp

osur

e Ra

te (u

R/h

r)

50

6 MV

10 MV

Background

(a)


0

20

40

60

80

100

120

140

160

0 10 20 30 40 50


Expo

sure

Rat

e (u

R/h

r) 6 MV

10 MV

Background

(b)

Fig. E.2. Skyshine exposure rates obtained at ground locations for 10 cm × 10 cm field size. Locations are ground level to the west of the accelerator facility. The gantry angles were (a) 180° and (b) 205°.

59

Appendix F – Skyshine Data and Analysis For Outdoor Roof Locations

60

Table F.1. Raw Data for skyshine measurements for outdoor roof locations Date: __3/12/08___________ All Readings are in Volts.

Location: _Roof at Outer Fence_____________ Reading 1 Reading 2 Reading 3 Reading 4 Reading 5


Gantry 180 degrees (vertical) Collimator 40x40 HIGH RATE


Collimator 10x10 HIGH RATE6 MV Photons 0.044 0.044 0.044 0.044 0.044

10 MV Photons 0.056 0.055 0.056 0.055 0.056

Location: _3 ft from fence_____________ Reading 1 Reading 2 Reading 3 Reading 4 Reading 5




Collimator 10x10 HIGH RATE6 MV Photons 0.039 0.038 0.039 0.038 0.039

10 MV Photons 0.048 0.049 0.048 0.049 0.049






HIGH RATE 10 MV Photons 0.042 0.043 0.042 0.042 0.043

Location: _9 ft from fence_____________





HIGH RATE 10 MV Photons 0.038 0.039 0.038 0.039 0.038

61

Table F.1 Location: _12 ft from fence____________





10 MV Photons 6.880 6.920 6.950 6.940 6.950






10 MV Photons 6.190 6.210 6.220 6.250 6.240






10 MV Photons 4.070 4.090 4.120 4.110 4.100






10 MV Photons 3.180 3.186 3.210 3.237 3.218

62

Table F.1 Location: _60 ft from fence_____________

Reading 1 Reading 2 Reading 3 Reading 4 Reading 5Background Readings: 0.275 0.284 0.288 0.286 0.279Collimator Closed 6 MVCollimator Closed 10 MV




10 MV Photons 2.578 2.582 2.589 2.570 2.574






10 MV Photons 2.088 2.085 2.090 2.097 2.095






10 MV Photons 1.792 1.802 1.801 1.778 1.752






10 MV Photons 1.554 1.561 1.565 1.567 1.573

63

Table F.1 Location: _120 ft from fence_____________





10 MV Photons 1.382 1.371 1.402 1.399 1.398






10 MV Photons 1.238 1.248 1.237 1.272 1.281

Date: 4/8/08Location: _At middle fence___________





10 MV Photons 0.343 0.343 0.341 0.342 0.343

Location: _9 ft from middle fence (15.5 ft from inner fence)__________ Reading 1 Reading 2 Reading 3 Reading 4 Reading 5





10 MV Photons 0.125 0.124 0.125 0.125 0.124

64

Table F.1 Location: 14 ft from middle fence (20.5 ft from inner fence)___________





10 MV Photons 0.087 0.085 0.085 0.086 0.087

Location: _Outside outer fence__________ Reading 1 Reading 2 Reading 3 Reading 4 Reading 5





10 MV Photons 0.055 0.055 0.055 0.055 0.055

65

Table F.2. Analysis of skyshine measurements for outdoor roof locations The low range calibration factor is 0.018147 V/(μR/h) and the high range calibration factor is 0.0000915 V/(μR/h).

Average Std Dev Avg Exposure Stdev NET Exp Stdev ExpLocation: _Roof at Outer Fence_____________ Isocenter dist = 11.92 metersBackground 0.302 0.011 16.631 1.775Leakage 6 MV 0.441 0.021 24.324 2.683 7.69 3.270Leakage 10 MV 0.449 0.007 24.731 2.502 8.10 3.123

HIGH RATE180 Open 6 MV 0.666 0.002 7274.317 727.620 7249.81 727.630180 Open 10 MV 0.834 0.001 9116.940 911.740 9092.02 911.747

HIGH RATE180 10x10 6 MV 0.044 0.000 480.874 48.087 456.36 48.235180 10x10 10 MV 0.056 0.001 607.650 61.059 582.73 61.167

Location: _3 ft from fence_____________ Isocenter dist = 12.835 metersBackground 0.314 0.010 17.314 1.814Leakage 6 MV 0.409 0.005 22.549 2.274 5.24 2.943Leakage 10 MV 0.461 0.016 25.393 2.686 8.08 3.272


HIGH RATE180 10x10 6 MV 0.039 0.001 421.858 42.608 399.81 42.751180 10x10 10 MV 0.049 0.001 531.148 53.451 506.25 53.584

E



180 10x10 6 MV 6.948 0.031 382.873 38.326 361.13 38.481180 10x10 10 MV 0.042 0.001 463.388 46.724 441.64 46.864



180 10x10 6 MV 6.222 0.008 342.867 34.290 321.89 34.46180 10x10 10 MV 0.038 0.001 419.672 42.392 396.37 42.54

66

Table F.2

Average Std Dev Avg Exposure Stdev NET Exp Stdev ExpLocation: _12 ft from fence____________ Isocenter dist = 15.579 metersBackground 0.303 0.013 16.708 1.813Leakage 6 MV 0.376 0.006 20.698 2.097 3.99 2.81Leakage 10 MV 0.395 0.007 21.745 2.209 5.04 2.89


180 10x10 6 MV 5.502 0.008 303.191 30.323 282.38 30.51180 10x10 10 MV 6.928 0.029 381.771 38.212 359.92 38.37



180 10x10 6 MV 4.928 0.041 271.560 27.249 253.11 27.45180 10x10 10 MV 6.222 0.024 342.867 34.312 322.48 34.49



180 10x10 6 MV 3.326 0.005 183.281 18.331 165.14 18.63180 10x10 10 MV 4.098 0.019 225.822 22.607 206.18 22.85



180 10x10 6 MV 2.601 0.013 143.352 14.354 124.07 14.73180 10x10 10 MV 3.206 0.023 176.679 17.715 158.12 18.03

67

Table F.2 Location: _60 ft from fence_____________ Isocenter dist = 30.213 metersBackground 0.282 0.005 15.562 1.584


180 10x10 6 MV 2.110 0.011 116.295 11.646 99.48 12.24180 10x10 10 MV 2.579 0.007 142.095 14.215 125.28 14.71

Location: _75 ft from fence_____________ Isocenter dist = 34.786 metersBackground 0.307 0.012 16.928 1.824


180 10x10 6 MV 1.727 0.018 95.178 9.567 78.36 10.282180 10x10 10 MV 2.091 0.005 115.226 11.526 98.41 12.125

Average Std Dev Avg Exposure Stdev NET Exp Stdev ExpLocation: _90 ft from fence_____________ Isocenter dist = 39.359 metersBackground 0.292 0.011 16.113 1.717


180 10x10 6 MV 1.485 0.005 81.810 8.185 64.99 8.396180 10x10 10 MV 1.785 0.021 98.363 9.903 81.55 10.078

Location: _105 ft from fence_____________ Isocenter dist = 43.932 metersBackground 0.293 0.014 16.124 1.782


180 10x10 6 MV 1.314 0.015 72.387 7.288 55.57 7.524180 10x10 10 MV 1.564 0.007 86.185 8.627 69.37 8.827 Location: _120 ft from fence_____________ Isocenter dist = 48.505 metersBackground 0.313 0.009 17.237 1.790


180 10x10 6 MV 1.158 0.013 63.790 6.419 46.97 6.685180 10x10 10 MV 1.390 0.013 76.619 7.697 59.80 7.921

68

Table F.2

Average Std Dev Avg Exposure Stdev NET Exp Stdev ExpLocation: _135 ft from fence_____________ Isocenter dist = 53.079 metersBackground 0.286 0.009 15.760 1.644


180 10x10 6 MV 1.054 0.005 58.081 5.814 41.26 6.11180 10x10 10 MV 1.255 0.020 69.168 7.006 52.35 7.25

Location: _At middle fence___________ Isocenter dist = 5.810 metersBackground 0.3022 0.0074 16.6529 1.714Leakage 6 MV 1.3136 0.0169 72.3866 7.298 55.73 7.533Leakage 10 MV 1.7396 0.0153 95.8616 9.623 79.21 9.803

180 Open 6 MV 4.3440 0.0167 47475.4098 4751.062 47467.40 4751.068180 Open 10 MV 5.4960 0.0055 60065.5738 6006.856 60057.16 6006.864

180 10x10 6 MV 0.2670 0.0007 2918.0328 291.906 2910.03 292.009180 10x10 10 MV 0.3424 0.0009 3742.0765 374.335 3733.66 374.468

Location: _9 ft from middle fence (15.5 ft from inner fence)__________ Isocenter dist = 8.646 metersBackground 0.2880 0.0137 15.8704 1.758Leakage 6 MV 0.6832 0.1393 37.6481 8.551 21.78 8.753Leakage 10 MV 0.7258 0.0172 39.9956 4.110 24.13 4.515

180 Open 6 MV 1.5582 0.0026 17029.5082 1703.186 17021.50 1703.209180 Open 10 MV 1.9566 0.0053 21383.6066 2139.137 21375.19 2139.143

180 10x10 6 MV 0.0986 0.0005 1077.5956 107.926 1069.59 108.296180 10x10 10 MV 0.1246 0.0005 1361.7486 136.306 1353.34 136.394

Location: 14 ft from middle fence (20.5 ft from inner fence)___________ Isocenter dist = 10.170 metersBackground 0.2994 0.0081 16.4986 1.709Leakage 6 MV 0.5120 0.0059 28.2140 2.840 11.72 3.399Leakage 10 MV 0.5740 0.0049 31.6306 3.175 15.13 3.684

180 Open 6 MV 1.0418 0.0011 11385.7923 1138.642 11377.79 1138.649180 Open 10 MV 1.3046 0.0018 14257.9235 1425.931 14249.51 1425.937

180 10x10 6 MV 0.0678 0.0004 740.9836 74.259 732.98 74.361180 10x10 10 MV 0.0860 0.0010 939.8907 94.622 931.48 94.712

Location: _Outside outer fence__________ Isocenter dist = 11.920 metersBackground 0.2934 0.0080 16.1680 1.676Leakage 6 MV 0.4322 0.0070 23.8166 2.413 7.65 3.051Leakage 10 MV 0.4674 0.0048 25.7563 2.589 9.59 3.193

180 Open 6 MV 0.6604 0.0027 7217.4863 722.352 7209.48 722.361180 Open 10 MV 0.8306 0.0021 9077.5956 908.042 9069.18 908.050

180 10x10 6 MV 8.8020 0.0130 485.0388 48.509 477.03 48.641180 10x10 10 MV 0.0550 0.0000 601.0929 60.109 592.68 60.223

69

In Table F.2, “Average” and “Std Dev” refer to the PIC voltage measurements. The

“Avg Exposure” is the average exposure rate (in μR/h) calculated using the appropriate

calibration factor. The “Stdev” is the standard deviation of the average exposure rate

taking into account the variation in the data and the 10% uncertainty in the calibration

factor. “NET Exp” is obtained by subtracting net leakage and average background

exposure rates from the average exposure rate for the location. “Stdev Exp” takes into

account the propagation of error from the measurement, the calibration factors, the

leakage and the background.

70

Appendix G – Skyshine Data and Analysis For Second Floor Corridor Locations

71

Table G.1. Raw Data for skyshine measurements for second floor corridor locations

All Readings are in Volts.Date: _3/5/08___________Location: At wall

Reading 1 Reading 2 Reading 3 Reading 4 Reading 5Background Readings: 0.343 0.339 0.352 0.356 0.345

Beam Energy = 6 MVCollimator Wide Open

Gantry: 180° 2.122 2.146 2.170 2.175 2.158155º 5.290 5.300 5.320 5.300 5.290

Collimator 10×10Gantry: 180° 0.450 0.457 0.439 0.460 0.453

155º 0.676 0.659 0.649 0.668 0.670


Gantry: 180° 2.882 2.898 2.897 2.895 2.887155º 7.530 0.037 HR 0.036 HR 0.036 HR 0.037 HR

0.038 0.037 0.036 0.036 0.037Collimator 10×10

Gantry: 180° 0.506 0.515 0.499 0.486 0.492155º 0.808 0.828 0.837 0.832 0.810


Background Readings: 0.364 0.335 0.336 0.332 0.334


Gantry: 180° 1.559 1.602 1.635 1.675 1.661155º 3.690 3.710 3.690 3.650 3.680


155º 0.532 0.555 0.559 0.551 0.552


Gantry: 180° 2.153 2.161 2.156 2.159 2.141155º 5.030 5.060 5.050 5.080 5.100


155º 0.664 0.670 0.669 0.665 0.673

Location: 30 feet from wall Reading 1 Reading 2 Reading 3 Reading 4 Reading 5



Gantry: 180° 0.918 0.884 0.924 0.893 0.937155º 1.485 1.512 1.484 1.472 1.501


155º 0.358 0.386 0.389 0.395 0.388


Gantry: 180° 1.149 1.161 1.160 1.151 1.160155º 1.944 1.943 1.953 1.962 1.959


155º 0.416 0.406 0.410 0.441 0.431

72

Table G.1 Location: 45 ft from wall

Reading 1 Reading 2 Reading 3 Reading 4 Reading 5Background Readings: 0.307 0.310 0.305 0.306 0.297


Gantry: 180° 0.678 0.687 0.693 0.709 0.708155º 0.935 0.917 0.910 0.900 0.903


155º 0.348 0.347 0.348 0.351 0.343


Gantry: 180° 0.325 0.819 0.812 0.802 0.808155º 1.129 1.143 1.144 1.125 1.122


155º 0.387 0.382 0.407 0.406 0.381


Background Readings: 0.317 0.308 0.305 0.300 0.245Beam Energy = 6 MVCollimator Wide Open

Gantry: 180° 0.505 0.508 0.515 0.519 0.533155º 0.566 0.567 0.567 0.574 0.591


155º 0.337 0.342 0.353 0.338 0.320


Gantry: 180° 0.599 0.594 0.601 0.609 0.613155º 0.720 0.697 0.698 0.699 0.716


155º 0.337 0.319 0.329 0.347 0.340




Gantry: 180° 0.547 0.550 0.550 0.532 0.544155º 0.601 0.596 0.607 0.599 0.590


155º 0.325 0.386 0.344 0.332 0.331


Gantry: 180° 0.630 0.628 0.624 0.637 0.657155º 0.696 0.716 0.716 0.707 0.697


155º 0.342 0.350 0.361 0.349 0.335

73

Table G.1




Gantry: 180° 0.493 0.515 0.506 0.524 0.521155º 0.535 0.561 0.557 0.546 0.528


155º 0.305 0.303 0.316 0.334 0.344


Gantry: 180° 0.601 0.593 0.598 0.584 0.581155º 0.603 0.608 0.617 0.623 0.625


155º 0.329 0.343 0.346 0.337 0.334




Gantry: 180° 0.421 0.425 0.423 0.430 0.439155º 0.437 0.453 0.443 0.437 0.447


155º 0.336 0.332 0.339 0.342 0.331


Gantry: 180° 0.477 0.482 0.493 0.510 0.499155º 0.506 0.503 0.514 0.510 0.510


155º 0.324 0.327 0.334 0.345 0.337

74

Table G.2. Analysis of skyshine measurements for second floor corridor locations

Avg Reading Std Dev Avg Exposure NET Exposure Std Dev ExpDistance = 0 ft μR/h μR/h μR/h

Background 0.347 0.007 19.12


180° 2.154 0.021 118.71 101.18 10.37155º 5.300 0.012 292.06 274.53 27.62

Collimator 10×10 180° 0.452 0.008 24.90 7.37 0.87155º 0.664 0.011 36.61 19.08 2.06


180° 2.892 0.007 159.35 141.83 14.33

155º 0.037 0.001 402.11 384.59 35.06Collimator 10×10

180° 0.500 0.011 27.53 10.00 1.16155º 0.823 0.013 45.35 27.82 2.96

Distance=15 ft

Background 0.340 0.013 18.75


180° 1.626 0.047 89.62 72.10 7.91155º 3.684 0.022 203.01 185.48 18.81

Collimator 10×10 180° 0.420 0.010 23.16 5.63 0.70155º 0.550 0.010 30.30 12.77 1.43


180° 2.154 0.008 118.70 101.17 10.26155º 5.064 0.027 279.05 261.53 26.48

Collimator 10×10 180° 0.436 0.007 24.01 6.49 0.78155º 0.668 0.004 36.82 19.29 2.06

Distance=30 μR/h μR/h μR/h

Background 0.314 0.009 17.31


180° 0.911 0.022 50.21 32.68 3.52155º 1.491 0.016 82.15 64.62 6.66

Collimator 10×10 180° 0.351 0.010 19.35 1.83 0.30155º 0.383 0.014 21.12 3.59 0.51


180° 1.156 0.006 63.71 46.19 4.76155º 1.952 0.009 107.58 90.05 9.15

Collimator 10×10 180° 0.376 0.009 20.74 3.21 0.45155º 0.421 0.015 23.19 5.66 0.73

75

Table G.2 Distance=45

Background 0.305 0.005 16.81


180° 0.695 0.013 38.30 20.77 2.25155º 0.913 0.014 50.31 32.78 3.46

Collimator 10×10 180° 0.325 0.012 17.90 0.37 0.12155º 0.347 0.003 19.14 1.62 0.26


180° 0.713 0.217 39.30 21.77 7.87155º 1.133 0.010 62.41 44.88 4.65

Collimator 10×10 180° 0.358 0.013 19.73 2.20 0.35155º 0.393 0.013 21.63 4.11 0.56

Distance=60 μR/h μR/h μR/h

Background 0.295 0.029 16.26Beam Energy = 6 MVCollimator Wide Open

180° 0.516 0.011 28.43 10.91 1.24155º 0.573 0.011 31.58 14.05 1.56

Collimator 10×10 180° 0.334 0.020 18.42 0.89 0.22155º 0.338 0.012 18.63 1.10 0.22


180° 0.603 0.008 33.24 15.71 1.71155º 0.706 0.011 38.90 21.38 2.30

Collimator 10×10 180° 0.347 0.012 19.12 1.59 0.28155º 0.334 0.011 18.43 0.90 0.19

Distance=75

Background 0.328 0.017 18.09


180° 0.545 0.007 30.01 12.48 1.39155º 0.599 0.006 32.99 15.46 1.68

Collimator 10×10 180° 0.335 0.011 18.45 0.92 0.20155º 0.344 0.025 18.93 1.41 0.32


180° 0.635 0.013 35.00 17.48 1.92155º 0.706 0.010 38.93 21.40 2.29

Collimator 10×10 180° 0.347 0.019 19.11 1.58 0.31155º 0.347 0.010 19.14 1.62 0.28

76

Table G.2

Avg Reading Std Dev Avg Exposure NET Exposure Std Dev ExpDistance=90 μR/h μR/h μR/h

Background 0.307 0.009 16.90


180° 0.512 0.013 28.20 10.68 1.23155º 0.545 0.014 30.05 12.53 1.43

Collimator 10×10 180° 0.333 0.010 18.33 0.80 0.18155º 0.320 0.018 17.66 0.13 0.08


180° 0.591 0.009 32.59 15.06 1.65155º 0.615 0.009 33.90 16.37 1.79

Collimator 10×10 180° 0.318 0.009 17.55 0.02 0.02155º 0.338 0.007 18.61 1.09 0.21

Distance=105

Background 0.308 0.012 16.99


180° 0.428 0.007 23.56 6.04 0.73155º 0.443 0.007 24.43 6.91 0.82

Collimator 10×10 180° 0.323 0.010 17.79 0.26 0.09155º 0.336 0.005 18.52 0.99 0.19


180° 0.492 0.013 27.12 9.60 1.12155º 0.509 0.004 28.03 10.50 1.18

Collimator 10×10 180° 0.324 0.009 17.84 0.32 0.10155º 0.333 0.008 18.37 0.84 0.18

77

78

Appendix H

Comparison of Skyshine to Primary Beam

The primary beam measurements are from the radiation survey performed 9/17/2007. The dose rate setting was 400 MU/min (400 cGy/min) for the radiation survey and the maximum collimator setting was 35 cm × 35 cm. In order to compare the skyshine radiation exposure rates to the primary beam exposure rates, the primary beam was scaled by a factor of 600 cGy min-1/400 cGy min-1 to account for the different dose rates. The skyshine radiation was scaled by a factor of (35cm)2/(40cm)2 to account for the different beam sizes. Primary Beam measured with Victoreen 451 P Primary Beam Scaled to Gantry 270 400 cGy/min 600 cGy/min mR/hr mR/hr Collimator 6 MV 10 MV Collimator 6 MV 10 MV 35 x 35 2.4 14.6 35 x 35 3.6 21.9 10 x 10 1.2 5.5 10 x 10 1.8 8.25 Skyshine measured with RSS-120 Gantry 180 600 cGy/min Scale 40 x 40 to 35 x 35 mR/hr mR/hr Collimator 6 MV 10 MV Collimator 6 MV 10 MV 40 x 40 1.88 2.32 35 x 35 1.44 1.78 10 x 10 0.115 0.144 10 x 10 0.088 0.110 Ratios of Skyshine to Primary Beam Collimator 6 MV 10 MV 35 x 35 0.40 0.08 10 x 10 0.05 0.01

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