PURPOSE: To assess the risk of a secondary malignant neoplasm (SMN) from proton 
therapy relative to intensity-modulated radiation therapy (IMRT) using X-rays, 
taking into account contributions from both primary and secondary sources of 
radiation, for prostate cancer.
METHODS AND MATERIALS: A proton therapy plan and a 6-MV IMRT plan were 
constructed for 3 patients with early-stage 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 from Monte Carlo simulations and available measured 
data, respectively. The risk of an SMN was 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.