VMC + + Validation for photon beams in the energy range of 20–1000 keV

Purpose: 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. Methods: Dose calculations were performed in different 40 × 40 × 40 cm 3 phantoms of different materials. Dose distributions of monoenergetic (ranging from 20 to 1000 keV) 10 × 10 and 2 × 2 cm 2 parallel beams were calculated. Voxel sizes of 4 × 4 × 4 and 1 × 1 × 1 mm 3 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. Results: At energies between 100 and 1000 keV, EGSnrc and VMC + + calculated dose distributions agree within the statistical uncertainty of about 1% ( 1 σ ) . At energies ≤ 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. Conclusions: 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.