GEANT4 for dosimetric GEANT4 for dosimetric study of an intracavitary study of an intracavitary brachytherapy applicatorbrachytherapy applicator

Emily PoonEmily PoonFrank VerhaegenFrank Verhaegen

March 6, 2006March 6, 2006

McGill UniversityMcGill UniversityMontreal, CanadaMontreal, Canada

22

Ir-192 HDR BrachytherapyIr-192 HDR Brachytherapy3.5 mm

5 mm

1.1 mm

2 m

0.6 mm

Nucletron HDR ‘classic’ model

We generated a phsp file for40 million photons reaching the capsule surface in a vacuum.

33

Validation of TG-43 parametersValidation of TG-43 parameters

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 30 60 90 120 150 180theta (degrees)

F(r

,th

eta)

r = 0.25 cm r = 0.5 cm r = 1 cm

r = 2 cm r = 3 cm r = 5 cm

Radial dose function Anisotropy function

0.5

0.6

0.7

0.8

0.9

1.0

1.1

0 2 4 6 8 10 12 14radial distance (cm)

rad

ial

do

se f

un

ctio

n g

(r)

GEANT4

Williamson and Li (1995)

(b)

0.5

0.6

0.7

0.8

0.9

1.0

1.1

0 2 4 6 8 10 12 14radial distance (cm)

rad

ial

do

se f

un

ctio

n g

(r)

GEANT4

Williamson and Li (1995)

Agreement: within 0.5% Agreement: within 2 %

44

Modeling of rectal applicatorModeling of rectal applicator

2 cm • made of silicone rubber• 8 catheters for HDR 192Ir source• allows for insertion of shielding

shielding made of lead or tungsten

55

Applicator with balloonApplicator with balloon

water or contrast medium

• protection for healthy tissue• contrast medium for dose reduction and better localization of balloon

tumor

balloon withiodine solution

66

Plato treatment planning systemPlato treatment planning system

CT-based CT-based dose calculations dose calculations

according to TG-43according to TG-43 assumes assumes

homogeneous water homogeneous water mediummedium

does not account for does not account for applicator and applicator and patient anatomypatient anatomy

77

TG-43 vs dose kernel calculations

Lead shielding

dose kernel:

• 100x100x40 voxels (10x10x10 cm3)• computed using DOSXYZ because GEANT4 is too slow

TG-43

Dose kernel

88

no shielding lead shielding

3-D patient calculations using dose kernels

50 %100 %300 %

99

GEANT4 simulationsGEANT4 simulations

• Low energy model • Photon transport only• Kerma calculations using track length estimation

1010

Dose around the tip regionDose around the tip region

no shielding tungsten shielding

1111

Dose around the balloonDose around the balloon

no shielding tungsten shielding

1212

Experimental validationExperimental validation

Good agreement between GEANT4 and GafChromic EBT film measurements

no shielding tungsten shielding

Solid lines: GEANT4dotted lines: EBT

1313

no shielding tungsten shielding

• conformal distributions can be attained by proper selection of source positions and dwell times• tungsten shielding offers significant radiation protection

1414

Ion chamber measurementsIon chamber measurements

Extradin A14P chamber

192Ir source

variable thickness

30 x 30 x 30 cm3 Lucite phantom

1515

GEANT4 vs ion chamber measurementsGEANT4 vs ion chamber measurements

Ion chamber GEANT4

• Ion chamber: high uncertainties in partially shielded regions

DoseW/DosenoW

1616

Speed issuesSpeed issues

Number of voxelsNumber of voxels Time/history (ms)Time/history (ms)

125125 0.8150.815

10001000 2.232.23

125000125000 120120

CPU time for a 2.4 GHz processor to simulate a photon history in a 30x30x40 cm3 water phantom

GEANT4 is too slow for patient calculations!

1717

Boundary crossing problemsBoundary crossing problems We use track-length estimator We use track-length estimator

for kerma calculationsfor kerma calculations Dose dependent on photon Dose dependent on photon

step size step size When voxels are constructed When voxels are constructed

as segments of a sphere, as segments of a sphere, some photons cross the some photons cross the boundaries without stoppingboundaries without stopping Errors in calculationsErrors in calculations Error is larger when Error is larger when θθ spans a spans a

smaller anglesmaller angle

g(r) of an isotropic 192Ir point source

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

1.05

1.10

0 3 6 9 12 15

radial distance (cm)

g(r

)

∆θ = 1 deg

∆θ = 3 deg

∆θ = 180 deg

1818

User code…User code…

void Sphere01VoxelParameterisation::ComputeDimensions(G4Sphere& voxel,const G4int copyNo, const G4VPhysicalVolume*) const{ voxel.SetInsideRadius(rInner[copyNo]); voxel.SetOuterRadius(rOuter[copyNo]); voxel.SetStartPhiAngle(0.*deg); voxel.SetDeltaPhiAngle(360.*deg); voxel.SetStartThetaAngle(87.5*deg); voxel.SetDeltaThetaAngle(5.*deg);}

We have an isotropic point source originating from (0,0,0)

Voxels are constructed as shown below:

In this case, all photons reaching the voxel regions should have θ between 87.5º and 92.5º.

Phantom is homogeneous water

1919

User code (cont’d)User code (cont’d)We set the voxels as “sensitive detector” (SD)

As a test: in “ProcessHits” function of the user SD class, we recorded the pre-step and post-step positions of photons entering the voxels

G4ThreeVector preStepPos=aStep->GetPreStepPoint()->GetPosition();G4ThreeVector postStepPos=aStep->GetPostStepPoint()->GetPosition();

preStepPos.theta() and postStepPos.theta() should be between 87.5º and 92.5º In a test run of 1 million histories, 0.6% of the photons crossed the boundaries

2020

ConclusionsConclusions Dosimetric properties of a novel intracavitary brachytherapy applicator have been studied. GEANT4 results are in good agreement with GafChromic EBT film and ion chamber measurements. A phsp file of the 192Ir source that we generated using GEANT4 will be used in another code (to be developed) for fast Monte Carlo calculations. Speed and some boundary crossing issues need to be addressed.