GEANT4 for GEANT4 for dosimetric study of dosimetric study of an intracavitary an intracavitary brachytherapy brachytherapy applicator applicator Emily Poon Emily Poon Frank Verhaegen Frank Verhaegen March 6, 2006 March 6, 2006 McGill University McGill University Montreal, Canada Montreal, Canada
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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. - PowerPoint PPT Presentation
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GEANT4 for dosimetric GEANT4 for dosimetric study of an intracavitary study of an intracavitary brachytherapy applicatorbrachytherapy applicator
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
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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
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no shielding lead shielding
3-D patient calculations using dose kernels
50 %100 %300 %
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GEANT4 simulationsGEANT4 simulations
• Low energy model • Photon transport only• Kerma calculations using track length estimation
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Dose around the tip regionDose around the tip region
no shielding tungsten shielding
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Dose around the balloonDose around the balloon
no shielding tungsten shielding
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Experimental validationExperimental validation
Good agreement between GEANT4 and GafChromic EBT film measurements
no shielding tungsten shielding
Solid lines: GEANT4dotted lines: EBT
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no shielding tungsten shielding
• conformal distributions can be attained by proper selection of source positions and dwell times• tungsten shielding offers significant radiation protection
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Ion chamber measurementsIon chamber measurements
Extradin A14P chamber
192Ir source
variable thickness
30 x 30 x 30 cm3 Lucite phantom
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GEANT4 vs ion chamber measurementsGEANT4 vs ion chamber measurements
Ion chamber GEANT4
• Ion chamber: high uncertainties in partially shielded regions
DoseW/DosenoW
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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!
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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
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.