Experimental determination of organ-specific fast neutron equivalent dose from modern radiotherapy LINACs

The out-of-field neutron dose from high-energy medical LINACs is a key factor in assessing secondary cancer risk. While the neutron source strength ( Q ) has been characterized, data on the resulting organ-specific fast neutron equivalent dose ( H f a s t ) for modern LINACs is scarce. This study provides new experimental data on the fast neutron equivalent dose for the Varian TrueBeam and Elekta Versa HD, including the first determination of organ-specific doses for the TrueBeam under a clinical Intensity-Modulated Radiation Therapy (IMRT) plan. We performed measurements with Solid-State Nuclear Track Detector (SSNTD, CR-39 type) in an anthropomorphic phantom using 6 MV and 18 MV beams for both standard fields and a clinical breast IMRT plan. We calculated the equivalent dose using two distinct methodologies: Linear Energy Transfer (LET) spectrometry and a direct calibration factor approach. Our results quantify the significant dose variations between organs, driven by tissue composition, and confirm a measurable, non-trivial dose from 6 MV beams, which is relevant for high-dose stereotactic techniques. These novel experimental data provide essential benchmarks for validating the models used in clinical risk assessment. • First experimental organ-specific fast neutron dose data for a Varian TrueBeam. • Quantifies fast neutron dose for TrueBeam (6 & 18 MV) and Versa HD (18 MV) LINACs. • Compares two dosimetric methods: LET spectrometry and calibration factor. • Confirms measurable, non-trivial neutron dose from 6 MV radiotherapy beams. • Delivers essential benchmarks for patient secondary cancer risk assessment models.