Implementation of geometry modules into the Monte Carlo dose calculation algorithm VMCpro to modulate proton beams for the treatment planning of ocular tumours

In future, Monte Carlo MC algorithms can be used for dose calculation in proton beam treatment planning if they are fast enough. The recently developed MC code VMCpro simulates proton transport in human tissue using the condensed history technique with continuous energy loss, multiple scattering, energy straggling, ionization, nuclear interactions as well as secondary electron and proton transport. The cross sections and stopping powers are based on ICRU tabulations. For the treatment of ocular tumors at the Hahn Meitner Institute in Berlin, a proton beam of about 68 MeV is modulated using different range shifters, modulator wheels and collimators. To take these devices into account, MC geometry modules are developed for VMCpro. Range shifters are modeled as material slabs and modulator wheels are simulated as slabs with variable thickness. The particle transport algorithm through these slabs is similar to the transport algorithm within the patient. Collimators are modeled as block cut outs. The code is parallelized using the Message Passing Interface and installed on a computer cluster to allow dose calculations with 0.1 mm spatial resolution in a few minutes. Calculations of dose distributions in water are performed for different combinations of range shifters, modulator wheels and collimators. The results are compared to measurements and simulations using MCNPX and GEANT4 to verify the fast MC code including the new geometry modules