SU-E-T-502: Dose Perturbation Effects Near Implant Surfaces Caused by Secondary Electron Transport in Photon-Beam Therapy.

PURPOSE: To investigate the dose perturbation effects at interfaces between water and a Titanium implant, attributable to secondary electron transport across the interface, during high energy photon radiotherapy. While dose enhancement is characteristic for the proximal interface of a high-atomic number implant, the dose perturbation at the distal interface varies from reduction to enhancement, requiring proper computation of secondary electron transport effects. The backward and forward perturbation factors pb and pf will be calculated. METHODS: Using DOSRZnrc, depth dose curves were computed in a water phantom using photon spectra of nominal energies 4, 6, 10, 15, 24 MV for conditions (i) homogeneous water without any insert, (ii) alternatively with Titanium inserts of thicknesses 3 and 5 cm placed at 10 cm water depth. Backscatter factor pb was computed as the ratio of the dose with implant against that without implant, whereas pf was calculated by first accounting for photon attenuation in the implant and then taking the ratio of the dose with implant against that without implant. RESULTS: At the front interface, pb is independent of the material thickness and varies slightly with beam energy and incident angle. On consideration of photon attenuation in the implant, pf was also found to be independent on material thickness, but strongly varying with energy, including change of sign. CONCLUSIONS: For 4-24 MV photon beams the maximum spread of the dose perturbation effect remains within only a few millimeters from the interface, with pb values ranging from 1.18-1.22, while factor pf ranges from 0.9-1.21 at normal incidence, indicating the extent to which planning systems may over- or underestimate the doses near implant interfaces. At inclined beam incidence the dose perturbation effects even increase, and for instance pb (1.24-1.25) and pf (0.85-1.32) were determined for 6 MV and 24 MV beams at 45° incidence.