1. Emily Poon Frank Verhaegen March 6, 2006
GEANT4 for dosimetric study of an intracavitary brachytherapy applicator
Emily Poon
Frank Verhaegen
March 6, 2006
McGill University
Montreal, Canada

2. Ir-192 HDR Brachytherapy 3.5 mm 5 mm 1.1 mm 2 m 0.6 mm
Nucletron HDR ‘classic’ model
We generated a phsp file for 40 million photons reaching the capsule surface in a vacuum.

3. Validation of TG-43 parameters
0.5
0.6
0.7
0.8
0.9
1.0
1.1 2 4 6 8 10 12 14
radial distance (cm)
radial dose function g(r)
GEANT4
Williamson and Li (1995)
(b)
Radial dose function
Anisotropy function
Agreement: within 0.5%
Agreement: within 2 %

4. Modeling 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

5. Applicator with balloon
water or
contrast medium
tumor
balloon with
iodine solution
protection for healthy tissue
contrast medium for dose
reduction and better
localization of balloon

6. Plato treatment planning system
CT-based
dose calculations according to TG-43
assumes homogeneous water medium
does not account for applicator and patient anatomy

7. TG-43 vs dose kernel calculations
100x100x40 voxels
(10x10x10 cm3)
computed using DOSXYZ
because GEANT4 is too slow
Lead shielding

8. 3-D patient calculations using dose kernels
50 %
100 %
300 %
no shielding
lead shielding

9. GEANT4 simulations Low energy model Photon transport only
Kerma calculations using track length estimation

10. Dose around the tip region
no shielding
tungsten shielding

11. Dose around the balloon
no shielding
tungsten shielding

12. Experimental validation
Solid lines: GEANT4
dotted lines: EBT
no shielding
tungsten shielding
Good agreement between GEANT4 and GafChromic EBT film measurements

13. conformal distributions can be attained by proper selection
no shielding
tungsten shielding
conformal distributions can be attained by proper selection
of source positions and dwell times
tungsten shielding offers significant radiation protection

14. Ion chamber measurements
Extradin A14P chamber
192Ir source
variable thickness
30 x 30 x 30 cm3 Lucite phantom

15. GEANT4 vs ion chamber measurements
DoseW/DosenoW
Ion chamber
GEANT4
Ion chamber: high uncertainties in partially shielded regions

16. Speed issues Number of voxels Time/history (ms) 125 0.815 1000 2.23
125000
120
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!

17. Boundary crossing problems
We use track-length estimator for kerma calculations
Dose dependent on photon step size
When voxels are constructed as segments of a sphere, some photons cross the boundaries without stopping
Errors in calculations
Error is larger when θ spans a smaller angle

18. User code… We have an isotropic point source originating from (0,0,0)
Phantom is homogeneous water
Voxels are constructed as shown below:
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); }
In this case, all photons reaching the voxel regions should have θ between 87.5º and 92.5º.

19. 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

20. Conclusions 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.