Room return effect of secondary neutrons generated by protons, helium, carbon and oxygen ions for radiotherapy

<i>Objective.</i>Secondary neutrons generated during ion beam radiotherapy undergo scattering from treatment room structures. However, their impact for different primary ion species remains insufficiently characterised. Therefore, this study aims to quantify the room return effect of secondary neutrons in radiotherapy for four different primary ion species, namely protons, helium, carbon and oxygen ions, with energies in the range relevant for radiotherapy.<i>Approach.</i>Ambient dose equivalent,<i>H</i>^*(10), was measured using three types of rem counters to characterise the neutron field generated by mono-energetic beams of increasing energy of the four primary ions. The rem counters were iteratively placed in four positions around a 30 cm × 30 cm × 30 cm RW3 phantom. Experimental data were compared to Monte Carlo (MC) simulation using a detailed room geometry. Next, the simulation was performed without the room to quantify the room return effect.<i>Main results.</i>MC simulations agreed with the experimental data within the uncertainty ranges.<i>H</i>^*(10) decreased with increasing angle relative to the beam direction but increased with higher primary beam energies. Among the ion species studied, oxygen produced the highest values of<i>H</i>^*(10) per primary particle, while protons produced the lowest. The room return effect was found to increase with both, the larger angles from the beam axis and the increasing ion energy, ranging from 17% up to 83% of the total<i>H</i>^*(10).<i>Significance.</i>This study presents the first quantitative assessment of the room return effect for four primary ion species, protons, helium, carbon, and oxygen, for clinically relevant energies. The results demonstrate that the treatment room itself plays a significant role for<i>H</i>^*(10), particularly through contributions from scattered secondary neutrons. Accurate modelling of the room geometry can help improve the reliability of MC simulations and reduce the risk of secondary neutron exposure misestimation during ion beam therapy.