Purpose: To investigate the dosimetric characteristics of I-125 seed model IAI-125A with Monte Carlo (MC) simulations using different physics models in Geant4 and compare the results and to previous publications.
Methods: A geometric model of IAI-125A seed was constructed with Geant4 v10.04 and FreeCAD. The source was placed in a 20X20X20 cm3 water box to simulate unbounded phantom conditions. The spectrum of Iodine was taken from National Nuclear Data Center, with mean photon energy of 28.36 keV. Two different physics models (default EM physics and Penelope physics) were used to simulate photo transport in water with 0.1 mm range cut to improve simulation efficiency. Command-based scoring method was used, with a cylindrical dose scoring mesh of radius 8.0 cm and height 8.0 cm. The number of bins in the radial/longitudinal direction is 400/160, giving a resolution of 0.2/0.5 mm. The number of histories used for each simulation is 1.5*1010 to obtain a relative uncertainty of ~1%. The dose matrices were exported and analyzed in Matlab
Results: The simulated radial dose function with default EM physics in Geant4 is different from AAPM consensus and other work using different MC software, which are also different from the AAPM consensus data(by 5% -10%). The maximum difference from AAPM consensus reaches 19% at 4cm radius. The MC results with Penelope for both radial dose function and anisotropy function are very close to one of previous publications (the difference for radial dose function is less than 1%), hence confirm their work.
Conclusion: Our study shows that the default EM physics in Geant4 might not be suitable for dosimetric study of low energy seeds. It is recommended that AAPM consensus data be updated to recent MC simulations and Penelope may be used in Geant4 for the dosimetric study of low energy sources.