VMC++: Validation of the Monte Carlo code for low energies

Abstract

In teletherapy VMC++ is known to be a very accurate and efficient Monte Carlo (MC) code. In principle, the MC method is also a powerful dose calculation tool in Brachytherapy or for orthovoltage radiotherapy. However, VMC++ is not validated for the low energy range of this application. Thus, this work aims in the validation of the VMC++ MC code for photon beams in the low energy range, i.e. between 20 and 1000 keV. Dose calculations were performed in a 40x40x40 cm3 water phantom with two 4 cm thick slabs of bone and lung. Dose distributions of mono-energetic (ranging from 20 - 1000 keV) 10x10 cm 2 sized parallel beams as well as a 10x10 cm2 sized parallel beam using the energy spectrum for Iridium- 192 were calculated. A voxel size of 4x4x4 mm3 was used for all dose calculations. The resulting dose distributions were compared with those calculated using EGSnrc, which is used as golden standard in this work. At energies between 100 keV and 1000 keV, EGSnrc and VMC++ calculated dose distributions agree within a statistical uncertainty of about 1% (1σ). At energies ≤ 50 keV beams, local dose differences for doses > 10% of Dmax of up to 4% occur when VMC++ and EGSnrc are compared. Turning off Rayleigh scattering, binding effects for Compton scattering and the atomic relaxation after photoelectric absorption in EGSnrc (not implemented in VMC++) leads to an agreement between both MC codes within 2% (local dose difference). Although further improvements for very low energies in accuracy of VMC++ could be achieved by implementing these interactions, this MC Code is able to calculate dose distributions for Ir-192 brachytherapy within statistical uncertainty. © 2009 Springer-Verlag.