Evaluation of shielding materials against photoneutrons in radiotherapy through Monte Carlo simulation with FLUKA

High-energy linear accelerators (>10 MV) used in radiotherapy generate unwanted photoneutrons through photonuclear reactions. This study evaluates the effectiveness of different shielding materials to reduce photoneutron production while preserving therapeutic beam characteristics.

A detailed Monte Carlo model of a Varian Clinac 2100C linear accelerator was developed using FLUKA version 4-5.0. The geometry was validated by comparing simulated depth-dose distributions and dose profiles with experimental measurements. Four moderating materials (plexiglass, polyethylene, titanium hydride TiH₂, zirconium hydride ZrH₂) combined with boron-10 as an absorber were evaluated in three thickness configurations (2 + 1, 3.5 + 2.5, 5 + 4 cm) across three detection regions.

Geometric validation demonstrated excellent agreement with experimental measurements (gamma pass rates 3 %/3 mm: 97-100 %). Titanium hydride proved to be the most effective moderator, achieving neutron flux reductions of 88.3 % in the proximal region and up to 88.1 % at the phantom surface in the thickest configuration. Attenuation efficiency increases with thickness and distance from the source. The impact on the therapeutic beam remains moderate, with a maximum photon fluence reduction of 41 % for TiH₂.

This study demonstrates the feasibility of significant photoneutron reduction through strategic implementation of moderator-absorber systems in the accelerator head, paving the way for optimized radiation protection in high-energy radiotherapy.