Out-of-field Dosimetry for Secondary Cancer Studies: Past Experience and Future Needs

Out-of-field Dosimetry for Out-of-field Dosimetry for Secondary Cancer Studies: Past Secondary Cancer Studies: Past Experience and Future NeedsExperience and Future Needs

David Followill, Ph.D.David Followill, Ph.D.

Radiological Physics CenterRadiological Physics Center

U. T. M. D. Anderson Cancer CenterU. T. M. D. Anderson Cancer Center

IntroductionIntroduction

As we have all known for a long time:As we have all known for a long time:Patients undergoing radiation therapy are Patients undergoing radiation therapy are exposed to secondary radiation (radiation out exposed to secondary radiation (radiation out of the treatment field).of the treatment field).

Secondary radiation is composed of photons, Secondary radiation is composed of photons, and at high treatment energies (above 8 and at high treatment energies (above 8 MV), neutrons, which are produced in the MV), neutrons, which are produced in the accelerator head.accelerator head.

Introduction - PhotonsIntroduction - PhotonsSecondary photon radiation composed Secondary photon radiation composed

of scatter and leakage.of scatter and leakage.

Scatter from within patient and off of collimators Scatter from within patient and off of collimators is dominant source near the treatment field.is dominant source near the treatment field.

Leakage through the accelerator head is the Leakage through the accelerator head is the dominant source away from the treatment field.dominant source away from the treatment field.

Leakage and Scatter from machine

Scatter from within patient

Introduction - NeutronsIntroduction - NeutronsNeutrons are produced primarily by Neutrons are produced primarily by photons striking the primary collimator, photons striking the primary collimator, jaws, and target.jaws, and target.

Neutrons are important because of their Neutrons are important because of their high RBE. high RBE. ????????????????

Radiation Type Energy Quality Factor

X and gamma Rays All 1

< 10 keV 510 keV to 100 keV 10100 keV to 2 MeV 202 MeV to 20 MeV 10

> 20 MeV 5

Neutrons

Why all this Concern?Why all this Concern?There are now new treatment techniques and There are now new treatment techniques and devices being used:devices being used:– Proton machinesProton machines– Tomotherapy unitsTomotherapy units– CyberKnife unitsCyberKnife units– IMRT deliveryIMRT delivery

The new treatment techniques and/or devices The new treatment techniques and/or devices are designed to deliver high dose gradients such are designed to deliver high dose gradients such that the target gets a high dose and the that the target gets a high dose and the surrounding normal tissues get a lower dose.surrounding normal tissues get a lower dose.The great dose distributions sometimes come at The great dose distributions sometimes come at a cost!a cost!

Why all this Concern?Why all this Concern?Amount of secondary radiation is a function of the Amount of secondary radiation is a function of the amount of beam-on time.amount of beam-on time.Some IMRT treatments may require up to 4 times Some IMRT treatments may require up to 4 times as many MU’s to deliver as conventional as many MU’s to deliver as conventional treatments.treatments.For deep treatment sites, low energy treatments For deep treatment sites, low energy treatments typically require more MU’s than high energy typically require more MU’s than high energy treatments.treatments.High energy x-rays and protons produce neutronsHigh energy x-rays and protons produce neutrons

Bottom LineBottom Line: More MU’s mean more : More MU’s mean more secondary radiation. secondary radiation.

Why are we Concerned?Why are we Concerned?

Hall 2006

LNT – BEIR VIILNT – BEIR VIILinear Exponential (Gray 1965, Schneider et al. 2005)Linear Exponential (Gray 1965, Schneider et al. 2005)– model suggested from human, animal, and model suggested from human, animal, and in vitroin vitro data data

Linear Plateau (Ron 1998)Linear Plateau (Ron 1998)– derived from human epidemiological studies of radiation-derived from human epidemiological studies of radiation-

induced breast, bladder, and stomach cancers induced breast, bladder, and stomach cancers

This is what started it ALLThis is what started it ALL

Technique 6 MV 18 MV 25 MV

Conventional 0.3 1.8 3

MLC modulated

1.0 5.1 8.4

Serial Tomotherapy

2.7 14.9 24.4

Calculated Risk estimates Calculated Risk estimates Followill et al (1997)Followill et al (1997)

Likelihood of Secondary Fatal Malignancy (%)

Where are we Concerned?Where are we Concerned?Secondary radiation in the “Secondary radiation in the “Patient PlanePatient Plane”!!”!!

Let’s First Worry about PhotonsLet’s First Worry about Photons

Early measurements – early 80’sEarly measurements – early 80’s– Ion chambers in large water phantomsIon chambers in large water phantoms

Large volume ion chambers (0.3 – 30 cc)Large volume ion chambers (0.3 – 30 cc)

Scanning tanksScanning tanks

More MeasurementsMore MeasurementsPhantoms began to more closely Phantoms began to more closely approximate actual patient geometryapproximate actual patient geometry– Using cylindrical ion chambersUsing cylindrical ion chambers

More MeasurementsMore MeasurementsSolid geometric phantoms also usedSolid geometric phantoms also used– Using TLD, diodes and 0.6 cmUsing TLD, diodes and 0.6 cm33 ion ion

chamberschambers

Mutic et al (1998)

More MeasurementsMore MeasurementsSolid geometric phantoms also usedSolid geometric phantoms also used– Using cylindrical small volume ion chambersUsing cylindrical small volume ion chambers

Klein et al. (2006)

Most Recent MeasurementsMost Recent MeasurementsAnthropomorphic Rando phantom with TLD -100 Anthropomorphic Rando phantom with TLD -100 and 700 depending on x-ray energy at 10 specific and 700 depending on x-ray energy at 10 specific organ sitesorgan sites..

3 TLD at each location.3 TLD at each location.

Kry et al (2005)

Adult ProcedureAdult ProcedureAdult Prostate Treatment with Adult Prostate Treatment with TomoTherapy and CyberKnife unitsTomoTherapy and CyberKnife units– Same prescription for all treatment devicesSame prescription for all treatment devices– Common TLD placement in phantom organ locations Common TLD placement in phantom organ locations

(2 TLDs)(2 TLDs)

Pediatric ProceduresPediatric ProceduresPediatric TomoTherapy Cranio-Spinal Irradiation Pediatric TomoTherapy Cranio-Spinal Irradiation (CSI) and CyberKnife GBM treatment(CSI) and CyberKnife GBM treatment– Same prescription for Same prescription for

3D and Tomotherapy treatment plans 3D and Tomotherapy treatment plans

IMRT and CyberKnife treatment plansIMRT and CyberKnife treatment plans

– TLD and EBT film placement in pediatric phantomTLD and EBT film placement in pediatric phantom– Organ doses from TLD-100Organ doses from TLD-100– EBT film validation of TPS calculationsEBT film validation of TPS calculations

Photon Measurement CautionsPhoton Measurement Cautions

1.1. Biggest issue: low doses = very low rdgs.Biggest issue: low doses = very low rdgs.– Increase in uncertainty of measurementsIncrease in uncertainty of measurements– Long exposure timesLong exposure times

2.2. Need for multiple rdgs. at each point.Need for multiple rdgs. at each point.

3.3. Rdg. location (air vs. phantom) Rdg. location (air vs. phantom) or at what depth?or at what depth?

4.4. Point vs. volume measurements.Point vs. volume measurements.

5.5. Neutron component for high X-ray energiesNeutron component for high X-ray energies

Photon Dose Equivalent as a percent of dose at dmax vs. Distance

Out of Field Photon Dose for Varian Accelerators

0.01

0.1

1

0 10 20 30 40 50 60

Distance From Central Axis (cm)

Dose a

s P

ercent

of

Dm

ax o

n C

entr

al A

xis

6 MV V

10 MV V

15 MV V

18 MV V

Mutic/Klein

Stern

Neutron MeasurementsNeutron MeasurementsNeutron fluence measured with gold foils.Neutron fluence measured with gold foils.– 197197Au(n,Au(n,))198198AuAu

Count the Count the emissions of the foils, convert to emissions of the foils, convert to neutron fluence by NIST traceable conversion neutron fluence by NIST traceable conversion factor: factor: Gold foils detect thermal neutrons.Gold foils detect thermal neutrons.– Bare gold foils measured the thermal neutron fluence.Bare gold foils measured the thermal neutron fluence.– Fast neutrons are thermalized by moderators. Gold Fast neutrons are thermalized by moderators. Gold

foils placed in moderators thereby measure the fast foils placed in moderators thereby measure the fast neutron fluence. neutron fluence.

Neutron MeasurementsNeutron MeasurementsDetermining neutron Determining neutron dose equiv. comprises dose equiv. comprises several stepsseveral steps– Obtain NIST traceable Obtain NIST traceable

calibrationcalibration– Measure neutron fluenceMeasure neutron fluence– Calculate neutron dose Calculate neutron dose

equivalent at dequivalent at dmaxmax

– Calculate neutron dose Calculate neutron dose equivalent at depthequivalent at depth

Neutron MeasurementsNeutron MeasurementsBonner sphere system to measure the Bonner sphere system to measure the fluence from which the neutron spectrum fluence from which the neutron spectrum is deconvolved.is deconvolved.

Howell et al (2006)

Neutron MeasurementsNeutron MeasurementsBubble detectors or neutron metersBubble detectors or neutron meters

Neutron Contribution (%) to Neutron Contribution (%) to Secondary DoseSecondary Dose

18 MV C 10 MV V 15 MV V 15 MV S 18 MV VColon 20 0.7 23 13 43Liver edge 28 1.1 28 16 49Stomach edge 28 1.3 30 18 54Liver center 36 1.1 35 21 58Stomach center 38 1.2 35 22 59Esophagus edge 31 0.7 29 18 54Lung edge 55 2.3 52 29 72Lung center 55 1.3 46 22 71Esophagus center 58 1.2 49 23 72Thyroid 80 6.1 74 36 85Bone Marrow 50 3.3 53 32 70

Organ SitePercent of Total Dose Equivalent from Neutrons

Data from S. Kry

Neutron FluenceNeutron FluenceFast neutron fluence measured on CAX and out of field.Fast neutron fluence measured on CAX and out of field.

Fast neutron fluence out of field varied by less than the uncertainty Fast neutron fluence out of field varied by less than the uncertainty in the dosimeter. Fast neutron fluence assumed constant out of in the dosimeter. Fast neutron fluence assumed constant out of field.field.

For each distance from central axis, the neutron fluence was broken For each distance from central axis, the neutron fluence was broken down into 12 components to account for energy and geometry.down into 12 components to account for energy and geometry.

Neutron fluence was examined at the same 10 points as where the Neutron fluence was examined at the same 10 points as where the photon dose was measured. photon dose was measured.

Neutron Measurement CautionsNeutron Measurement Cautions1.1. Gold foil activation – not for everyone.Gold foil activation – not for everyone.

– NIST traceabilityNIST traceability– Still the “gold” standardStill the “gold” standard

2.2. Low doses = very low rdgs.Low doses = very low rdgs.– Increase in uncertainty of measurementsIncrease in uncertainty of measurements– Long exposure timesLong exposure times

3.3. Need for multiple rdgs. Along patient plane.Need for multiple rdgs. Along patient plane.

4.4. Measurement variability among the different Measurement variability among the different neutron dosimetersneutron dosimeters

5.5. Difficulty measuring the neutron dose at depth in Difficulty measuring the neutron dose at depth in a patienta patient

Dose Equivalent per Complete Dose Equivalent per Complete Prostate TreatmentProstate Treatment

(photon and neutron)(photon and neutron)

Dose Equivalent (cSv) per complete Adult Prostate Treatment18 MV 6 MV 6 MV 6 MV 18 MV

Organ Site 3D CRT IMRT TomoTherapy CyberKnife IMRTThyroid 12.7 5.3 2.4 34.4 54.6

Lung center 12.6 6.7 5.1 27.0 44.7Esophagus center 9.6 6.2 4.2 25.4 35.1

Liver center 24.2 15.5 12.2 27.7 69.3Stomach center 23.2 19.9 15.6 30.3 68.7

Trans. Colon 48.2 33.3 27.6 42.8 101.4Treatment Plan. Sys. Pinnacle Pinnacle HiArt plan station Multiplan Corvus

Dose Equivalent (cSv) per complete Adult Prostate Treatment18 MV 6 MV 6 MV 6 MV 18 MV




Trans. Colon 48.2 33.3 27.6 19.0 - 42.8 101.4Treatment Plan. Sys. Pinnacle Pinnacle HiArt plan station Multiplan Corvus

Data from Kry et al, S. Lazar, M. Bellon

Risk (%) of Secondary Cancer per Risk (%) of Secondary Cancer per Complete Prostate TreatmentComplete Prostate Treatment

(photon and neutron)(photon and neutron)

Data from Kry et al, S. Lazar, M. Bellon

Lifetime Risk (%) of Secondary Cancer per Prostate Treatment18 MV 6 MV 6 MV 6 MV 18 MV




Trans. Colon 0.24 0.16 0.14 0.21 0.50Treatment Plan. Sys. Pinnacle Pinnacle HiArt plan station Multiplan Corvus

Lifetime Risk (%) of Secondary Cancer per Prostate Treatment18 MV 6 MV 6 MV 6 MV 18 MV




Trans. Colon 0.24 0.16 0.14 0.09 - 0.21 0.50Treatment Plan. Sys. Pinnacle Pinnacle HiArt plan station Multiplan Corvus

Dose Equivalent to Edge of StomachDose Equivalent to Edge of StomachDose Equivalent to Center of Stomach

0

100

200

300

400

500

600

700

800

900

1000

18 MV CRTPinnacle

6MV VCorvus

6MV VPinnacle

6MV SCorvus

10MV VCorvus

15MV VCorvus

15MV SCorvus

18MV VCorvus

6MV TomoPlan. Sta.

6MV CKMultiplan

Treatment Approach

Do

se E

qu

ival

ent

(mS

v)

Neutron Dose Equivalent

Photon Dose Equivalent

Dose Equivalent to Center of Stomach

0

100

200

300

400

500

600

700

800

900

1000

18 MV CRTPinnacle

6MV VCorvus

6MV VPinnacle

6MV SCorvus

10MV VCorvus

15MV VCorvus

15MV SCorvus

18MV VCorvus

6MV TomoPlan. Sta.

6MV CKMultiplan

Treatment Approach

Do

se E

qu

ivale

nt

(mS

v)

Neutron Dose Equivalent

Photon Dose Equivalent

Dose Equivalent and Risk (%) per Dose Equivalent and Risk (%) per Complete Pediatric GBM TreatmentComplete Pediatric GBM TreatmentDose Equivalent (cSv) per complete Pediatric GBM Treatment

6 MV 6 MV CyberKnifeOrgan Site IMRT version 1.5 version 2.1

Thyroid 5.5 32.2 19.3Lung center 3.7 60.2 39.1

Breast 2.5 12.3 8.0Liver center 1.5 9.2 6.0Trans. Colon 1.1 8.5 5.5

Ovary 1.0 8.1 5.3Treatment Plan. Sys. Pinnacle Multiplan Multiplan

New optimization and tuning tools as well dose homogeneity

Lifetime Risk (%) per complete Pediatric GBM Treatment

6 MV 6 MV CyberKnifeOrgan Site IMRT version 1.5 version 2.1

Thyroid 0.04 0.24 0.15Lung center 0.10 1.59 1.03

Breast 0.05 0.26 0.17Liver center 0.01 0.03 0.02

Stomach center 0.02 0.12 0.08Ovary 0.00 0.04 0.02

Treatment Plan. Sys. Pinnacle Multiplan Multiplan

Now let’s include Proton TreatmentsNow let’s include Proton Treatments

Seems that every other day there is a new proton facility being built

Is there any Secondary Is there any Secondary Radiation to worry about?Radiation to worry about?

Proton beams generate Proton beams generate neutronsneutrons by by interacting with the scattering interacting with the scattering

systems, range modulator wheel, systems, range modulator wheel, collimators and even the patientcollimators and even the patient

YES!

How significant can this be?How significant can this be?

Hall (2006)

Stray Radiation Exposure from Different RT Facilities

0.00

0.01

0.10

1.00

10.00

100.00

1000.00

0 50 100 150

Distance from Field Edge (cm)

H/D

(m

Sv/

Gy)

Hall, Harvard, Passive,Normalized to 10x10 cm2Yan, Harvard, Passive, 8 cmSOBP, FS=5x5 cm2Zheng, PTCH, Passive, 8 cmSOBP, FS=10x10 cm2Hall, 4 Field CRT, 6 MV

Zheng, PTCH, Passive, PristinepeakHall, IMRT, 6 MV

Schneider, PSI, Scanning,Pristine peakMesolaras, MRPI, Passive

Polf, Harvard, Passive, LargeField, 8 cm SOBPTayama, PMRC, Passive, 10 cmSOBP, FS=11 cm2Fontenot, Harvard, Radiosurgery,pristine peakYan, Harvard, Passive,RadiosurgeryYan, Harvard, Ocular, 2.3 cmSOBP

Slide coutesy of J Fontenot

Neutron MeasurementsNeutron MeasurementsMeasured and calculated data is sparseMeasured and calculated data is sparse

Results are not consistentResults are not consistent

Measurement techniques are not consistentMeasurement techniques are not consistent

Many different factors affect neutron productionMany different factors affect neutron production– Machine type (synchrotron vs cyclotron)Machine type (synchrotron vs cyclotron)– Proton energyProton energy– Range modulationRange modulation– Field sizeField size– Lateral scattering techniqueLateral scattering technique

Bonner Sphere ExtensionBonner Sphere Extension

Slide courtesy of Rebecca Howell

•The BSS and BSE response function from thermal to 15 The BSS and BSE response function from thermal to 15 MeV is verified and corrected using AmBe & Cf-252 MeV is verified and corrected using AmBe & Cf-252 source at source at Georgia Tech.Georgia Tech.•The response function from 15 MeV up to 800 MeV is The response function from 15 MeV up to 800 MeV is corrected using the 800 MeV neutron beam at LANSCEcorrected using the 800 MeV neutron beam at LANSCE

23 cm23.5 cm Shoulder

Hip

Head

Experimental Setup

Neutron Measurement CautionsNeutron Measurement Cautions

1.1. Neutron energies much higher than Neutron energies much higher than observed around electron acceleratorsobserved around electron accelerators

2.2. Is your neutron calibration technique Is your neutron calibration technique calibrated appropriately for these calibrated appropriately for these “neutrons”?“neutrons”?

3.3. Long exposure timesLong exposure times

4.4. Moderators used on electron accelerators Moderators used on electron accelerators are not adequate.are not adequate.

Comparison of the doses in the pediatric CranioSpinal case Comparison of the doses in the pediatric CranioSpinal case treated by 3D-conventional, tomotherapy, and proton therapytreated by 3D-conventional, tomotherapy, and proton therapy

Organ site 3D 6 MV (cGy)

TomoTherapy (cGy)

Proton (120-180 MeV)

(cSv)

Thyroid 2797 (12) 362 (12) 22

Lt. Breast Bud 152 (9) 437 (6)

Heart center 2957 (41) 865 (16) 21.8

Heart edge 2345 (18) 438 (16)

Lt. Lung center 226 (13) 907 (43)

Lt. Lung Edge 242 (33) 446 (9)

Liver Center 2583 (18) 1107 (124)

Liver edge 217 (14) 545 (16)

Lt. Kidney 221 (6) 748 (83)

Bladder 195 (11) 77 (2) 17.7

Pelvic 86 (4) 528 (30)

Lt. Ovary 322 (60) 135 (22)Data from S. Lazar and Z. Wang, MDACC

SummarySummaryNo consensus as to the best measurement No consensus as to the best measurement technique. Calculations will play a larger role in the technique. Calculations will play a larger role in the future.future.Increases in the secondary dose is highly Increases in the secondary dose is highly dependent on the number of MUs and photon dependent on the number of MUs and photon energy.energy.Measurement of the secondary dose requires an Measurement of the secondary dose requires an established NIST traceable technique and established NIST traceable technique and characterization of the dosimeters at the characterization of the dosimeters at the appropriate energies.appropriate energies.New treatment planning software with better New treatment planning software with better optimization routines have reduced the number of optimization routines have reduced the number of MUs per treatment reducing the secondary doses.MUs per treatment reducing the secondary doses.Neutron dose may play less of a role than Neutron dose may play less of a role than previously thought.previously thought.

Take Home MessageTake Home MessageMeasurement techniques are maturing.Measurement techniques are maturing.

Patient models and dose calculations are Patient models and dose calculations are more sophisticated and accurate.more sophisticated and accurate.

Biggest uncertainty is the RISK estimateBiggest uncertainty is the RISK estimate

There are many factors to be considered There are many factors to be considered in the treatment of a patient and the risk of in the treatment of a patient and the risk of a secondary cancer is only one of many.a secondary cancer is only one of many.

I believe this risk to be small regardless of the treatment technique

Documents

Out-of-field Dosimetry for Secondary Cancer Studies: Past Experience and Future Needs