VMC++ validation for photon beams in the energy range of 20-1000 keV

In high energy 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 other areas in radiation oncology, e.g., brachytherapy or orthovoltage radiotherapy. However, VMC++ is not validated for the low-energy range of such applications. This work aims in the validation of the VMC++ MC code for photon beams in the energy range between 20 and 1000 keV.

Dose calculations were performed in different 40 x 40 x 40 cm3 phantoms of different materials. Dose distributions of monoenergetic (ranging from 20 to 1000 keV) 10 x 10 and 2 x 2 cm2 parallel beams were calculated. Voxel sizes of 4 x 4 x 4 and 1 x 1 x 1 mm3 were used for the dose calculations. The resulting dose distributions were compared to those calculated using EGSnrc, which is used as a golden standard in this work.

At energies between 100 and 1000 keV, EGSnrc and VMC++ calculated dose distributions agree within the statistical uncertainty of about 1% (1sigma). At energies < or = 50 keV, dose differences of up to 1.6% (in % of D(max)) 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 (all not implemented in VMC++) leads to an agreement between both MC codes within statistical uncertainty. Further, using the KERMA approximation feature implemented in VMC++ leads to very efficient simulations in the energy range between 20 and 1000 keV.

Further improvements for very low energies in accuracy of VMC++ could be achieved by implementing Rayleigh scattering, binding effects for Compton scattering, and the atomic relaxation after photoelectric absorption. Implementation into VMC++ of KERMA approximation has been validated.