Evaluation of neutron dose equivalent in prostate stereotactic body radiotherapy by bubble detector

Stereotactic body radiotherapy (SBRT) for prostate cancer often uses high-energy photon beams, which can produce photoneutrons through interaction in the linac head, raising concerns regarding neutron-related exposure. Therefore, it is important to estimate neutron dose equivalent in SBRT plans when comparing beam energies and treatment delivery techniques. BD-PND fast neutron detectors were used in this study to quantify neutron dose equivalent in prostate SBRT plans. We performed dosimetry measurements using a Varian TrueBeam Linac and assessed neutron production at energies of 2.5 MV, 6 MV, 6 MV flattening filter free (FFF), 10 MV, 10 MV FFF and 15 MV. Bubble counts were converted to neutron dose equivalent using the manufacturer-provided calibration factor. The measured neutron dose equivalent was highest at the centre and decreased at peripheral measurement locations farther from the primary beam. Significant differences in neutron dose equivalent were found between flattening filter (FF) and FFF beams at the central beam position after multiple comparison correction. In contrast, peripheral positions showed no statistically significant differences. Additionally, low bubble counts were observed under 6 MV and 6 MV FFF beams, suggesting a low-level detector response under the present measurement conditions. Overall, beam energy, FF or FFF delivery and measurement positions all affect the measured neutron dose equivalent in prostate SBRT. The low-energy observation should be interpreted as preliminary, and additional work is needed, including repeated measurements, different detector types and more clinically relevant measurement locations.