Dose calculations with the BEAM Monte Carlo code at extended SSD's

Extended Surface to Skin Distance (SSD) beam data are very useful for hemibody and total body irradiation. Monte Carlo simulation provides an alternative mechanism to calculate the dose for such treatment modalities. Comparison of the Monte Carlo results has been done with measured data of two linear accelerators of 6 MV, with results from two commercial 3D treatment-planning systems and with extrapolated data from measurements at 100 cm SSD. The results from the Monte Carlo simulations agreed with the experimental results within 1% and the PDD curves produced from the TPS's were within 2% error. The extrapolated PDD curves had a deviation of about 2%. It was found that d_max migrated towards the surface with increasing SSD which was predicted by the simulations. Also, the mean photon energy as computed by the simulation was found to be higher at extended SSD which is part of the reason why the PDDs are more penetrating for extended distances.