Linear accelerator photon beam interaction with the virtual physiological human

We have successfully used Monte Carlo (MC) techniques to simulate the geometry of a linear accelerator (LINAC) and the photon beam interaction with the virtual physiological humanoid (VPH) head. We used EGSnrc software to perform the MC simulations. The accelerator geometry and materials were based on the manufacturer's specifications for Varian-CLINAC 2300 CD. In order to verify the accuracy of the simulation, the photon beam interaction with a water phantom was simulated. The percent depth dose (PDD) and beam profiles of the simulated beams were compared with experimental measurements. Dosxyznrc was used to model the interaction of this beam with a 3-D CT phantom reconstructed from TIFF images of the Virtual Physiological Humanoid (VPH). A 3-D dose distribution was obtained and superimposed onto the phantom to obtain the isodose curves. The dose distributions were compared to those obtained by treatment planning software developed at the University of North Carolina (PlanUNC version 6.7.01R). It was found that MC methods gave generally lower entrance and exit doses than PlanUNC with percentage differences of 25 % and 10 % relative to maximum dose. Generally, we obtained similar dose distributions except in regions of significant inhomogeneity such as bone-soft tissue interfaces. Disagreements between MC simulation and treatment planning software results in the skin dose and in inhomogeneous regions can have significant clinical implications.