Measuring neutron spectra in radiotherapy using the nested neutron spectrometer

Area of Science:

• Medical Physics
• Radiation Detection and Measurement
• Radiotherapy Physics

Background:

• Photonuclear interactions in linear accelerators generate out-of-field neutron doses, posing risks in radiotherapy.
• Traditional neutron quantification methods, like Bonner sphere systems (BSS), are cumbersome and time-consuming.
• Accurate neutron dosimetry is essential for patient safety and occupational health in radiation oncology.

Purpose of the Study:

• To evaluate the performance of a novel nested neutron spectrometer (NNS) for in-room neutron measurements in radiotherapy bunkers.
• To compare the NNS's capabilities against traditional Bonner sphere systems (BSS) in terms of practicality and speed.
• To validate NNS-measured neutron spectra and dose equivalents using Monte Carlo simulations and bubble detector measurements.

Main Methods:

• The nested neutron spectrometer (NNS) was tested in a Varian Clinac 21EX radiotherapy bunker, operating in current-mode to avoid pulse pileup.
• Performance was assessed based on reproducibility, linearity, and dose-rate effects.
• Neutron spectra were measured at various locations, unfolded using a custom algorithm, and compared to Monte Carlo simulations and bubble detector measurements.

Main Results:

• NNS-measured neutron spectra showed expected variations with distance and location, transitioning from fast to thermal neutron dominance.
• Agreement between NNS measurements and Monte Carlo simulations was within 30-40%.
• Calculated neutron ambient dose equivalents from NNS data were consistent with bubble detector measurements.

Conclusions:

• The nested neutron spectrometer (NNS) provides a reliable and practical method for measuring neutron spectra in radiotherapy environments.
• The NNS allows for rapid spectral measurements (under 1 hour), including setup and data unfolding.
• This represents a significant advancement for neutron dosimetry and radiation protection in radiotherapy settings.