The risk of second primary cancers due to peripheral photon and neutron doses received during prostate cancer external beam radiation therapy.

Background: Out-of-field organs can be affected by secondary radiations originating from high energy linear accelerators, leading to an increased risk of carcinogenesis. The aim of this work was to determine the risk of second primary cancers (SPC) in the organs distal to the prostate during 3D conformal radiotherapy.

Materials And Methods: Based on previously measured peripheral photon and neutron doses in a Rando phantom using an 18MV photon beam, SPC risks in the out-of-field organs were estimated using the linear-no-threshold and the competitive risk models. Whole body as well as organ specific risk coefficients were used to calculate the SPC risks in order to estimate upper and lower risk limits, given the uncertainties associated with the coefficients.

Results: The corresponding estimated average SPC risks ranged from 1.5±0.3% for thyroid to 4.5±4.2% for colon using whole body risk coefficients and 0.12±0.03% and 1.45±1.34%, respectively, using organ specific risk coefficients. The linear-no-threshold and the competitive risk models resulted in the same risk estimates (within the estimated errors) in the dose range received by out-of-field organs (≤1Gy). Distally located organs such as lungs, oesophagus, and thyroid received higher neutron versus photon dose.

Conclusions: The findings have important radiation protection implications when using high energy linear accelerators, as radiation protective measures could be employed to minimize the secondary out-of-field radiation for patients undergoing high energy external beam irradiation of the prostate.