Dose Perturbations at Heterogeneous Interfaces in Radiotherapy - an EGSnrc based Monte Carlo Investigation

Radiotherapy dose computation in the presence of tissue heterogeneities presents a challenge for the treatment planning system (TPS). This study presents a Monte Carlo (MC) investigation of dose perturbation due to various kinds of heterogeneities (soft tissue interfaces with bone, air, lung and metallic implants) for a range of clinically useful photon energies and field sizes. EGSnrc MC radiation transport code has been used for linear accelerator head modeling and phantom dose calculations. Dose perturbations were quantified in terms of forward- and backscatter factors as well as inside the heterogeneity of low densities for various field sizes including 5 x 5 cm(2), 10 x 10 cm(2) and 15 x 15 cm(2). For lung heterogeneity, both under-dosage and over-dosage has been assessed in relation to the field sizes and energies. These simulations confirm under-dosage in the case of lung heterogeneity for small field sizes, which increases with the energy. However, lung tissues reflect an overdose with smaller energies and larger field sizes.