Physics Contribution Risk of Secondary Malignant Neoplasms from Proton Therapyand Intensity-modulated X-ray Therapy for Early-stage Prostate Cancer

Purpose: To assess the risk of a secondary malignant neoplasm (SMN) from proton therapy relative to intensitymodulated radiation therapy (IMRT) using X-rays, taking into account contributions from both primary and secondary sources of radiation, for prostate cancer. Methods andMaterials: A proton therapy plan and a 6-MV IMRT plan were constructed for 3 patients with earlystage adenocarcinoma of the prostate. Doses from the primary fields delivered to organs at risk of developing an SMN were determined from treatment plans. Secondary doses from the proton therapy and IMRT were determined fromMonte Carlo simulations and available measured data, respectively. The risk of an SMNwas estimated from primary and secondary doses on an organ-by-organ basis by use of risk models from the Committee on the Biological Effects of Ionizing Radiation. Results: Proton therapy reduced the risk of an SMN by 26% to 39% compared with IMRT. The risk of an SMN for both modalities was greatest in the in-field organs. However, the risks from the in-field organs were considerably lower with the proton therapy plan than with the IMRT plan. This reduction was attributed to the substantial sparing of the rectum and bladder from exposure to the therapeutic beam by the proton therapy plan. Conclusions: When considering exposure to primary and secondary radiation, proton therapy can reduce the risk of an SMN in prostate patients compared with contemporary IMRT. ! 2009 Elsevier Inc.