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GEANT4 for dosimetric study of an intracavitary brachytherapy applicator

GEANT4 for dosimetric study of an intracavitary brachytherapy applicator. Emily Poon Frank Verhaegen March 6, 2006. McGill University Montreal, Canada. 3.5 mm. 1.1 mm. 0.6 mm. 5 mm. 2 m. Ir-192 HDR Brachytherapy. Nucletron HDR ‘classic’ model.

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GEANT4 for dosimetric study of an intracavitary brachytherapy applicator

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

  2. 3.5 mm 1.1 mm 0.6 mm 5 mm 2 m Ir-192 HDR Brachytherapy Nucletron HDR ‘classic’ model We generated a phsp file for40 million photons reaching the capsule surface in a vacuum.

  3. 1.1 1.1 (b) 1.0 1.0 0.9 0.9 radial dose function g(r) radial dose function g(r) 0.8 0.8 0.7 0.7 GEANT4 GEANT4 0.6 0.6 Williamson and Li (1995) Williamson and Li (1995) 0.5 0.5 0 0 2 2 4 4 6 6 8 8 10 10 12 12 14 14 radial distance (cm) radial distance (cm) Validation of TG-43 parameters Radial dose function Anisotropy function Agreement: within 0.5% Agreement: within 2 %

  4. 2 cm Modeling of rectal applicator • made of silicone rubber • 8 catheters for HDR 192Ir source • allows for insertion of shielding shielding made of lead or tungsten

  5. tumor balloon with iodine solution Applicator with balloon water or contrast medium • 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 dose kernel: TG-43 • 100x100x40 voxels • (10x10x10 cm3) • computed using DOSXYZ • because GEANT4 is too slow Dose kernel Lead shielding

  8. 50 % 100 % 300 % 3-D patient calculations using dose kernels 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. 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. Extradin A14P chamber 192Ir source variable thickness 30 x 30 x 30 cm3 Lucite phantom Ion chamber measurements

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

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

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

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