A comprehensive spectrometry study of a stray neutron radiation field in scanning proton therapy

Abstract

The purpose of this study is to characterize the stray neutron radiation field in scanning proton therapy considering a pediatric anthropomorphic phantom and a clinically-relevant beam condition. Using two extended-range Bonner sphere spectrometry systems (ERBSS), Working Group 9 of the European Radiation Dosimetry Group measured neutron spectra at ten different positions around a pediatric anthropomorphic phantom irradiated for a brain tumor with a scanning proton beam. This study compares the different systems and unfolding codes as well as neutron spectra measured in similar conditions around a water tank phantom. The ten spectra measured with two ERBSS systems show a generally similar thermal component regardless of the position around the phantom while high energy neutrons (above 20 MeV) were only registered at positions near the beam axis (at 0°, 329° and 355°). Neutron spectra, fluence and ambient dose equivalent, H (*)(10), values of both systems were in good agreement (<15%) while the unfolding code proved to have a limited effect. The highest H (*)(10) value of 2.7 μSv Gy(-1) was measured at 329° to the beam axis and 1.63 m from the isocenter where high-energy neutrons (E ⩾ 20 MeV) contribute with about 53%. The neutron mapping within the gantry room showed that H (*)(10) values significantly decreased with distance and angular position with respect to the beam axis dropping to 0.52 μSv Gy(-1) at 90° and 3.35 m. Spectra at angles of 45° and 135° with respect to the beam axis measured here with an anthropomorphic phantom showed a similar peak structure at the thermal, fast and high energy range as in the previous water-tank experiments. Meanwhile, at 90°, small differences at the high-energy range were observed. Using ERBSS systems, neutron spectra mapping was performed to characterize the exposure of scanning proton therapy patients. The ten measured spectra provide precise information about the exposure of healthy organs to thermal, epithermal, evaporation and intra-nuclear cascade neutrons. This comprehensive spectrometry analysis can also help in understanding the tremendous literature data based rem-counters while also being of great value for general neutron shielding and radiation safety studies.

Systematic out‐of‐field secondary neutron spectrometry and dosimetry in pencil beam scanning proton therapy

“…As in Ref. [13], an evaporation peak was resolved in beam direction despite the existence of a strong high-energy peak. In previous studies, 12 it was not possible to resolve the evaporation peak completely.…”

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 .…”

“…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.…”

“…[222][223][224][225][226][227] 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. 94,108,110,[227][228][229][230] Similar to standard Bonner sphere spectrometers, response functions for each detector-sphere combination must be determined, accounting for the high-Z shell. During a neutron measurement, the irradiation must be repeated with each detector-sphere combination, and data must be unfolded to determine a neutron spectrum.…”

Excessive applicator radiation leakage for a common therapeutic kilovoltage system

“…In the diagnostic and therapeutic setting, guidelines and national standards 7,13,14 suffice as a best practice compliance document which provides recommendations on methodologies to quantify leakage radiation and its permissible limit. Although there are many publication in the literature which assess out-of-field radiation doses for therapeutic high energy photons, [15][16][17][18] electrons, 19-21 protons [22][23][24][25] or neutrons 26,27 very few publications assess the issue within objective: The objective of this work is to characterise out-of-field leakage radiation emanating from clinical applicators of the WOmed T-200 kilovoltage therapy machine. methods: To identify points of leakage, radiosensitive film was affixed to the walls and base plate of each applicator.…”