Poster — Thur Eve — 17: Effect of Heterogeneities Due to Kilo‐Voltage Photon Beam Energy in Small‐Animal Irradiation: A Monte Carlo Evaluation

This study evaluated the effect of tissue heterogeneities due to photon beams in the kilo‐voltage energy range in small‐animal irradiation. Monte Carlo(MC) simulation (EGSnrc code) was used. Three MC mouse phantoms were generated from a single mouse CTimage set. These phantoms were generated by overriding the relative electron density of no voxels (heterogeneous), all voxels (homogeneous) and the bone voxels (bone homogeneous) to one. Phase‐space files of the 100 and 225 kVp photon beams produced by a small animal irradiator were generated using BEAMnrc. A 360 deg photon arc was simulated for treatment of the lung, and 3D dose calculations were carried out for the three phantom geometries. The resulting dose profiles for the different phantoms and beam energies were compared. It was found that the 225 kVp photon beams have a better conformai dose distribution than the 100 kVp. The bone doses in the heterogeneous mouse phantom were about 4 – 5 (100 kVp) and 2 (225 kVp) times higher when compared to the homogeneous phantom. However, the lungdose does not vary significantly between the heterogeneous, homogeneous and bone homogeneous phantoms for either the 100 or 225 kVp photon beams. We concluded that bone dose enhancement was found when 100 and 225 kVp photon beams were used in small‐animal irradiation. This dosimetric effect due to the presence of the bone heterogeneity was more significant than the lung heterogeneity, and such bone dose enhancement does not occur in the typical patient's radiotherapy using the MV photon beams.