LIMITATIONS OF A PENCIL BEAM APPROACH TO PHOTON DOSE CALCULATIONS IN LUNG-TISSUE

A common limitation in treatment planning systems for photon dose calc ulation is to ignore the impact on electron transport and photon scatt er from patient heterogeneities. The heterogeneity correction is often based on scaling operations along beam rays as for the method accordi ng to Bathe or the more novel approach of 1D convolutions along beam p aths applied in pencil-beam-based systems. The effects of the limitati on have been studied in a mediastinium geometry for a wide range of be am qualities by comparing the results from a pencil-beam-based treatme nt planning system with the results from Monte Carlo calculations. As expected, the deviations within unit-density volumes are small while d eviations in low-density volumes increase with increasing beam energy from approximately 3% for 4 MV to 14% for 18 MV x-rays as a result of increased electron disequilibrium.