Out-of-field thermal neutron characterization with thin 3D-silicon detectors in Varian TrueBeam and Elekta Synergy LINACs.

Neutron contamination near medical linear accelerators (LINACs) has raised increasing concerns regarding secondary risks for patients undergoing photon-based radiotherapy (RT) at energies above 6 MV.Objective.In this work the aim was to evaluate in real-time the secondary neutron fluence to compare the neutron production between two widely used LINACs (Varian TrueBeam and Elekta Synergy) operating at 15 MV. Thermal neutron contributions were measured in the treatment rooms using a 10 × 10 cm2field size, flattening filter, and doses ranging from 100 to 1000 monitor units (MU) and dose rates from 100 to 600 MU min-1.Approach.An ultra-thin (20μm) silicon detector with 3D electrodes, paired with a (45 ± 5)μm thick10B-enriched conversion layer was used to quantify the thermal neutron field in both LINAC treatment rooms. The thin active volume of 20μm thick offers a high gamma rejection factor (>10-8), which is crucial for effectively isolating neutron signals in high-gamma RT environments. A customized readout electronics was developed for online measurements. The detector responses were compared with those obtained with PHITS Monte Carlo simulations using an extended model of the TrueBeam LINAC head operating at 15 MV. This model, which includes detailed head shielding for the first time, was validated with percentage depth dose data.Main results.The findings demonstrated the ability of the sensors to characterize the thermal neutron field, achieving a detection efficiency of (1.53 ± 0.02)% at a 660 keV energy threshold and operating without interference from the high intense photon background. The results show that the secondary neutron field produced by the Varian TrueBeam was approximately four times higher than that of the Elekta Synergy.Significance.These findings highlight significant differences in neutron contamination between LINAC models, thus with implications for patient safety and shielding design in RT facilities.