Sensitivity analysis of neutron and photon equivalent doses in patient organs during 3D conformal radiotherapy using Monte Carlo simulations.

This study evaluates neutron and photon equivalent dose sensitivity in patient organs during prostate three-dimensional conformal radiotherapy (3DCRT) using Monte Carlo simulations. A validated Varian Clinac 2100C LINAC model and a hybrid XCAT adult male phantom were used with an 18 MV photon beam. Component-wise sensitivity analysis showed that removal of the flattening filter caused the largest increase in neutron doses, with ratios of 1.49 in the testes, 1.39 in the bladder, and 1.38 in the colon, indicating a dominant shielding role. The primary collimator also showed significant shielding, with neutron dose ratios up to 1.29 in the rectum. Treatment room shielding contributed up to 61% of the neutron dose in the brain via room-return effects. Photon doses decreased with increasing distance from the field, highlighting spatial dependence and implications for radiological protection.