Estimation of secondary cancer risks following rectal cancer photon and proton therapy: A computational phantom study

The aim of this work was to assess secondary cancer risks after rectal cancer intensity modulated radiation therapy (IMRT) and intensity modulated proton therapy (IMPT) using a computational phantom.

A computational male phantom was used to represent Thai rectal cancer patients. Ethos treatment planning system (TPS) was used to calculate the IMRT plan with 12 fields of 6 MV flattening filter free (FFF) photon beams, while matRad was employed to calculate IMPT plans with 2 (lateral and oblique), 3 and 5 fields. The IMPT plans were recalculated by Tool for Particle Simulation (TOPAS) Monte Carlo simulation to account for secondary radiations. The assessment of secondary cancer risks in terms of organ equivalent dose (OED) and lifetime attributable risk (LAR) was conducted using the mechanistic model.

The IMPT plans yielded lower radiation doses to the organs at risk (OARs) than the IMRT plan. The organ-specific LARs for the IMRT plan were 2.247 % for the colon, 0.098 % for the bowel and 0.038 % for the bladder, while the LARs for the IMPT plans ranged from 1.475 % to 1.537 % for the colon, 0.014 % to 0.016 % for the bowel, and 0.071 % to 0.092 % for the bladder. The colon was found to have the highest risk of developing secondary cancer. The risks of developing secondary bone and soft tissue sarcomas as well as secondary cancer in out-of-field organs were negligibly low except in the IMRT case.

Among the investigated IMRT and IMPT plans, the 2-field oblique IMPT plan yielded the lowest risk.