Fluorescent nuclear track detectors for out-of-field neutron dosimetry in proton therapy

Secondary neutrons are a major concern regarding side effects in ion beam therapy because they contribute to the out-of-field dose, particularly important for sensitive patient groups such as pregnant and pediatric patients. Measuring these neutrons is challenging because of their high kinetic energy, which is imparted to charged particles like fragments and recoil protons. In addition, accurate measurements require small detectors that ideally do not disturb the radiation field when measuring inside a phantom. Fluorescent Nuclear Track Detectors (FNTDs) have already shown promising results in ion beam dosimetry and the measurement in fast neutron fields. Given their high spatial resolution and sensitivity, FNTDs offer a promising approach for characterizing secondary neutron doses in complex radiation environments, such as those encountered in proton therapy.

Establish a methodology for estimating neutron-induced out-of-field dose inside a phantom. The focus is to discuss the technical requirements and present initial experimental results from a proton treatment plan.

The analysis workflow for determining dose equivalent with FNTDs is introduced, including intensity-to-linear energy transfer (LET) in water conversion and track polar angle corrections. FNTDs were placed inside RW3 and polymethyl methacrylate phantoms and irradiated with a proton spread-out Bragg peak (SOBP) plan. Experimental results from two downstream positions in each phantom were used to benchmark Monte Carlo simulations.

A polar angle correction function was established, indicating intensity corrections of approximately a factor of 2 at $20{}^{\circ }$ and up to a factor of 3.5 beyond $50{}^{\circ }$ . Furthermore, a few short-range high-LET tracks with a low probability of occurrence have been found. Despite accounting for only about $1\%$ of the total fluence, high-LET tracks can contribute more than $50\%$ of the total dose equivalent. When not considering these short-range tracks, the relative agreement in dose equivalent between simulations and experiments was within ( $1.10\pm 0.10$ ) to ( $1.49\pm 0.13$ ).

This work presents the first LET-based method using FNTDs to estimate out-of-field neutron dose for a proton SOBP plan, measured inside a phantom. Integrating this method into clinical workflows may improve out-of-field dose estimation for sensitive patient groups, such as pregnant or pediatric patients, by enabling prior dose assessments using anthropomorphic phantoms.