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Indra J. Das, PhD, FACRDepartment of Radiation OncologyIndiana University of School of Medicine & Midwest Proton Radiation Therapy Institute (MPRI), Indiana

Commissioning and Calibrating a Linear Accelerator – State-of-the-Art in 2010

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Preface

Med. Phys. 35(9), 4186-4214, 2008

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2008

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Planning for Commissioning Data

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Time= [(PDD + 5 profiles)/beam energy]x ( open + 4 wedges) x 60 points/scanx [(1 s/pts + (1s/movement and delay)]x (15 fields x 2 energies)

∼105 s∼ 30 h

Planning for Commissioning Time

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Rational For Commissioning Beam DataFirst, it is not evident that manufacturing procedures for all linear accelerators have produced a level of reproducibility acceptable for clinical use. For example, variations in beam parameters have been noted between beams with the same nominal energies.

Second, on-site changes made during installation and acceptance of the user’s accelerator e.g., changes in beam energy and/or profiles from beam steering will not be modeled in the golden data.

Third, the beam characteristics of the soft wedges are made by moving jaws that depend on the speed parameters of the jaws and a deviation at site could affect the beam profile of the soft wedge.

Fourth, although acceptable agreement with the golden data set may be found in individual checks, it may be that some clinical setups will have multiple errors, which combine to produce unacceptable results.

TG-106

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Hrbacek, et al Med. Phys. 34, 2917–2927, 2007.

Rational For Not Using Golden Data

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Hrbacek, et al “Quantitative evaluation of a beam-matching procedure using one dimensional gamma analysis,” Med. Phys. 34, 2917–2927, 2007.

Rational For Not Using Golden Data

|xi − x¯ | <∆, ∀ xi

∆ =? (0.5, 1.0 0r 2%)

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Time required for commissioning adual energy linear accelerator with photon and electron beam is

Answer: 4Reference: Das et al, TG-106, Med. Phys. 35(9), 4186-4214, 2008

Question

1. 1 day2. 3 days3. 1 week4. 4-6 weeks5. 2 months

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Standard chamber 10−1 cm3—The active volume for a standard Farmer-type ionization chamber is on average 0.6 cm3.

Minichamber 10−2 cm3—The active volume for a mini-ionization chamber is on average 0.05 cm3.

Microchamber 10−3 cm3—The active volume for a microionization chamber is on average 0.007 cm3

and ideally suited for small field dosimetry such as radiosurgery,gamma knife, CyberKnife, and IMRT

Definition of Detectors

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Water

Air

1 2 3 4 5

Setting Water Tank & Detector

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Know Your Connectors

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Understand Detector, Connector & Cable

See Poster:Srivastava et al, SU-GG-T-270, 2010

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Quality of Cables

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When setting ion chamber in water tank, the correct position of the chamber as viewed in water tank (as seen in figure) is:

Water

Air

1 2 3 4 5

Answer: 3Reference : Das et al, TG-106, Med. Phys. 35(9), 4186-4214, 2008

Question

1. Position # 12. Position # 23. Position # 34. Position # 45. Position # 5

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Setup and Possible Errors

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Electrometer Null Setting Cable subtraction Proper bias >300 V for ion chamber 100 V for diamond 0 v for all diodes

Proper gain Proper mode

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Choose Consistent & Correct Polarity

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Chambers & Gain

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Selection of detector for beam data

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XYZ

XY

Z

Scan Direction

Chamber Orientations

Radiation beam

Choice of Detector Orientation

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Detector Orientation

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0102030405060708090

100110

0 0.5 1 1.5 2

Rel

ativ

e Dos

e

Distance Off Axis (cm)

6MV, 2x2 cm2 field, Illustration of chamber volume effects

Diode, dmax

PTW Pinpoint, dmax

RK chamber, dmax

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The possible setup error that causes the photon beam dose profile in figure is due to:

0

10

20

30

40

50

60

70

80

90

100

-4 -3 -2 -1 0 1 2 3 4Distance (cm)

Rel

ativ

e D

ose 1.5 cm

10 cm20 cm

Answer: 3Reference: Das et al, TG-106, Med. Phys. 35(9), 4186-4214, 2008

Question

1. Gantry tilt

2. Collimator rotation

3. Tank arm tilt

4. Gantry and arm tilt5. Gantry tilt, collimator rotation and

tank arm tilt

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0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0

Depth (mm)

10

20

30

40

50

60

70

80

90

100

0

Dos

e (%

)

Surface & Buildup Dose, 6 MV

0.6 cc0.3 cc0.125 ccMarkusA16IC-4PinPointDiamondPFDSFD

Reference: Das et al, TG-106, Med. Phys. 35(9), 4186-4214, 2008

Answer: 2

Question

The accurate measurement of the buildup and surface dose could be achieved by a:

1. Diode detector2. Parallel plate chamber3. Cylindrical chamber4. Diamond detector5. PinPoint chamber

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0

50

100

150

200

250

-20 -15 -10 -5 0 5 10 15 20

Rel

ativ

e D

ose

Distance Off Axis(cm)

6 MV 60 Deg Wedge, 10 cm depth: water vs Profiler

Diode Array Profile

Water Profile

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0

20

40

60

80

100

120

140

160

180

200

-20 -15 -10 -5 0 5 10 15 20

Rel

ativ

e D

ose

Distance Off Axis(cm)

18 MV 60 Degree Wedge, 10 cm depth, 100 cm SAD: Water vs Profiler

Diode Array profile

Water Profile

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Scanning Speed

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Scanning Speed

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0

20

40

60

80

100

0 5 10 15 20 25 30 35

Perc

ent D

epth

Dos

e

Depth(cm)

Arithmetic Mean (AM) smoothing: 60 Degree Wedge PDD

UnsmoothedAM x 1AM x 2AM x 3AM x 5

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Electron beam depth doses shown in figure represents the problem of

0

0.2

0.4

0.6

0.8

1.0

1.2

0 2 4 6 8 10 12 14 16 18 20

Depth, z (cm)

Dep

th D

ose

(%)

6 MeV9 MeV

12 MeV

15 MeV

18 MeV

21 MeV

Answer: 4Reference: Das et al, TG-106, Med. Phys. 35(9), 4186-4214, 2008

Question

1. Noise in the cable2. Electrometer gain3. Bias on the electrometer4. Speed of scanning5. Tuning of accelerator

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Future of Beam Data Commissioning

Standardization of linear acceleratorsMonte Carlo based commissioning Newer Radiation Detectors & Cables Newer Scanning Systems Smart algorithms

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Aubin et al., Med Phys, 37(5), 2279-2288, 2010

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Monte Carlo Codes

Aubin et al., Med Phys, 37(5), 2279-2288, 2010

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Simulation of intensity at target

Aubin et al., Med Phys, 37(5), 2279-2288, 2010

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Simulated Profiles

Aubin et al., Med Phys, 37(5), 2279-2288, 2010

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Profiles for different fields

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Depth Dose Simulation

Aubin et al., Med Phys, 37(5), 2279-2288, 2010

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Detector Manufacturer Type volumeSFD Scanditronix Photon diode 1.7x10-5cm3

PFD Scanditronix Photon diode 1.9x10-4cm3

Exradin A-16 Standard Imaging Ion chamber 0.007cm3

Wellhofer-IC4 Scanditronix Ion chamber 0.40 cm3

Pinpoint PTW Ion chamber 0.015cm3

0.125cc PTW Ion chamber 0.125cm3

0.3cc PTW Ion chamber 0.3 cm3

0.6cc PTW Ion chamber 0.6 cm3

Diamond PTW Diamond 0.003cm3

Markus PTW Parallel plate 0.055cm3

Edge Detector Sun Nuclear Diode 10-5cm3

Other

Detectors

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Sensitivity vs Volume of Detectors

0

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130

140

1.0E-04 1.0E-03 1.0E-02 1.0E-01 1.0E+00

Volume (cm3)

Rela

tive s

ensit

ivity

PFD

SFD

A-16 PinPointMarkus

IC4

0.125cc

0.3cc

0.6 cc

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Beam Profile

0

10

20

30

40

50

60

70

80

90

100

110

0 10 20 30 40 50 60 70 80Distance (mm)

Dos

e (%

)

0.6 cc0.3 cc0.125 ccMarkusA16IC-4PinPointDiamondPFDSFD

Answer: 5Reference: Das et al, TG-106, Med. Phys. 35(9), 4186-4214, 2008

Question

Photon beam dose profiles taken with various detectors as shown in figure is possibly due to:

1. Speed of scanning2. Beam asymmetry3. Pb piece in the beam4. Hysteresis of scanning system5. Orientation of scanning detector

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Comparison of Large Tank and Small SRS Cylinder Tank for SRS, TMR & Profiles

ARM Inc., Port Saint Lucie, FL 34983

Moving Tank System for TMR

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No SSD to SAD calculations required, No cubic spline fit of a limited number of fixed data points needed

0

0.2

0.4

0.6

0.8

1

1.2

0

0.2

0.4

0.6

0.8

1

1.2

0 50 100 150 200 250 300 350

Calculated TPR

Measured TPR

Calculated TPR ~2% less at depth Cubic spline fit of 12 data points

Nikesch et al, CyberKnife Center , Palm Beach, FL

Direct TMR Data Acquisition

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Scanner Orientation Advantage

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Sun Nuclear 3D Scanner

2

1

1

32

1. Ring drive maintains consistent scanning direction

2. Diameter drive has maximum scanning range of 640mm

3. Vertical drive has maximum travel of 400mm

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Conclusions Golden Data should be taken as a

reference only Understand time and amount of data

to be taken View each parameters properly,

double check by another individual Use proper detector for each type of

data collection Set optimum speed for scanning, do

not rush

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-Conclusions

Understand the limits and measuring condition

Question every unusual data set Do not smooth data too much Write report for future reference Future technology & resources

could help commissioning simpler

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Thanks