Secondary neutron spectrum from 250‐MeV passively scattered proton therapy: Measurement with an extended‐range Bonner sphere system

Abstract

Purpose: Secondary neutrons are an unavoidable consequence of proton therapy. While the neutron dose is low compared to the primary proton dose, its presence and contribution to the patient dose is nonetheless important. The most detailed information on neutrons includes an evaluation of the neutron spectrum. However, the vast majority of the literature that has reported secondary neutron spectra in proton therapy is based on computational methods rather than measurements. This is largely due to the inherent limitations in the majority of neutron detectors, which are either not suitable for spectral measurements or have limited response at energies greater than 20 MeV. Therefore, the primary objective of the present study was to measure a secondary neutron spectrum from a proton therapy beam using a spectrometer that is sensitive to neutron energies over the entire neutron energy spectrum. Methods: The authors measured the secondary neutron spectrum from a 250-MeV passively scattered proton beam in air at a distance of 100 cm laterally from isocenter using an extended-range Bonner sphere (ERBS) measurement system. Ambient dose equivalent H*(10) was calculated using measured fluence and fluence-to-ambient dose equivalent conversion coefficients. Results: The neutron fluence spectrum had a high-energy direct neutron peak, an evaporation peak, a thermal peak, and an intermediate energy continuum between the thermal and evaporation peaks. The H*(10) was dominated by the neutrons in the evaporation peak because of both their high abundance and the large quality conversion coefficients in that energy interval. The H*(10) 100 cm laterally from isocenter was 1.6 mSv per proton Gy (to isocenter). Approximately 35% of the dose equivalent was from neutrons with energies ≥20 MeV. Conclusions: The authors measured a neutron spectrum for external neutrons generated by a 250-MeV proton beam using an ERBS measurement system that was sensitive to neutrons over the entire energy range being measured, i.e., thermal to 250 MeV. The authors used the neutron fluence spectrum to demonstrate experimentally the contribution of neutrons with different energies to the total dose equivalent and in particular the contribution of high-energy neutrons (≥20 MeV). These are valuable reference data that can be directly compared with Monte Carlo and experimental data in the literature. © 2014 Author(s)

AAPM TG158: Measurement and calculation of doses outside the treated volume from external‐beam radiation therapy

“…Like the photon–neutron spectra, the proton neutrons have a peak at a few hundred keV that is produced from evaporation processes and is isotropically generated as well as a low‐energy tail that extends down to thermal energies from room‐scattered neutrons. Uniquely, however, the proton–neutron spectra have a second peak that starts at around 20 MeV and extends up to the maximum proton beam energy . The high‐energy peak contains forward‐directed neutrons from direct (nucleon–nucleon) reactions .…”

“…Neutron fluence spectrum per unit lethargy per proton Gy to isocenter measured for three different proton beam lines. Data sets I and II (unpublished data from R. Howell, measured at same facility and with same technique as in Howell et al …”

“…High‐Z shells are typically employed to increase the sensitivity of Bonner spheres to high‐energy neutrons (> 20 MeV), such as those encountered around proton or carbon ion beams that extend up to hundreds of MeV . Use of these shells creates an extended‐range Bonner sphere spectrometer, which can be used in conjunction with any of the thermal neutron detectors described above to measure secondary neutrons from proton therapy . Similar to standard Bonner sphere spectrometers, response functions for each detector–sphere combination must be determined, accounting for the high‐Z shell.…”

AAPM TG 191: Clinical use of luminescent dosimeters: TLDs and OSLDs

“…Neither type of TLD is typically useful for measuring neutrons with energy above thermal. Because TLDs are only thermal neutron detectors, TLD‐600 or TLD‐600/TLD‐700 pairs must be used in conjunction with a neutron moderator — for example, a Bonner sphere . The response of the detectors within the moderator requires a detailed calibration (see Ref.…”

Measurement of the energy spectrum of secondary neutrons in a proton therapy environment

“…Monte Carlo simulations can play an important role here; however, it is necessary to benchmark the simulations with measurements. This requires spectrometry, as neutron dose conversion coefficients are strongly energy dependent and the neutron energy spectrum can extend from thermal energies (~10 -8 MeV) to up to hundreds of MeV [2]. The only neutron spectrometers capable of covering such a wide energy range are Bonner sphere spectrometers (BSS).…”