Investigation of neutron dose and secondary cancer risk in pelvic and brain radiotherapy with a high-energy LINAC

‎This work aimed to assess the secondary cancer risk for patients undergoing brain and pelvic radiotherapy with high-energy linear accelerators (LINAC)‎. ‎Photoneutrons are produced in the LINAC’s head when operating above 7 MeV and are not considered in radiotherapy treatment planning‎. ‎The MCNPX2.6 Monte Carlo code was used for LINAC’s head simulation‎. ‎The photon and neutron doses were calculated in tissues/organs of an International Commission on Radiological Protection (ICRP) male reference voxel phantom undergoing pelvic and brain radiotherapy‎. The results indicated that the neutron equivalent dose was higher for tissues/organs located close to skin and‎, ‎contrary to the photon equivalent dose‎, ‎did not decrease sharply for tissues/organs outside of the irradiation field‎. ‎Notably‎, ‎neutron equivalent dose distribution was almost homogenous in whole body and did not depend on the treatment type and location of target organ‎. ‎Therefore‎, ‎an undesirable dose was received by healthy tissues/organs‎, ‎leading to an increase in secondary cancer risk‎. ‎Based on the obtained results‎, ‎the neutron effective dose for the pelvic and brain treatment were 0.20 and 0.26 mSv.Gy-1‎, ‎respectively‎. ‎The results also indicated that maximum secondary cancer risk due to neutrons was for colon (0.026 %) in the pelvic treatment‎, ‎while in brain treatment‎, ‎it belongs to stomach (0.036 %) for a delivered dose of 70 Gy‎. ‎It is recommended that a mean neutron effective dose value of 0.23 mSv.Gy-1 can be considered in brain and pelvic treatment planning for evaluating the secondary cancer risk of high-energy LINAC radiotherapy‎.