Extended-range BSS, TEPCs, and the WENDI-II enable accurate measurements of stray neutrons while other rem-counters are not appropriate considering the high-energy range of neutrons involved in proton therapy.

Proton beam therapy has advantages in comparison to conventional photon radiotherapy due to the physical properties of proton beams (e.g. sharp distal fall off, adjustable range and modulation). In proton therapy, there is the possibility of sparing healthy tissue close to the target volume. This is especially important when tumours are located next to critical organs and while treating cancer in paediatric patients. On the other hand, the interactions of protons with matter result in the production of secondary radiation, mostly neutrons and gamma radiation, which deposit their energy at a distance from the target. The aim of this study was to compare the response of different passive dosimetry systems in mixed radiation field induced by proton pencil beam inside anthropomorphic phantoms representing 5 and 10 years old children. Doses were measured in different organs with thermoluminescent (MTS-7, MTS-6 and MCP-N), radiophotoluminescent (GD-352 M and GD-302M), bubble and poly-allyl-diglycol carbonate (PADC) track detectors. Results show that RPL detectors are the less sensitive for neutrons than LiF TLDs and can be applied for in-phantom dosimetry of gamma component. Neutron doses determined using track detectors, bubble detectors and pairs of MTS-7/MTS-6 are consistent within the uncertainty range. This is the first study dealing with measurements on child anthropomorphic phantoms irradiated by a pencil scanning beam technique.

Systematic 3D mapping of out-of-field doses induced by a therapeutic proton pencil scanning beam in a 300  ×  300  ×  600 mm water phantom was performed using a set of thermoluminescence detectors (TLDs): MTS-7 (LiF:Mg,Ti), MTS-6 (LiF:Mg,Ti), MTS-N (LiF:Mg,Ti) and TLD-700 (LiF:Mg,Ti), radiophotoluminescent (RPL) detectors GD-352M and GD-302M, and polyallyldiglycol carbonate (PADC)-based (CHO) track-etched detectors. Neutron and gamma-ray doses, as well as linear energy transfer distributions, were experimentally determined at 200 points within the phantom. In parallel, the Geant4 Monte Carlo code was applied to calculate neutron and gamma radiation spectra at the position of each detector. For the cubic proton target volume of 100  ×  100  ×  100 mm (spread out Bragg peak with a modulation of 100 mm) the scattered photon doses along the main axis of the phantom perpendicular to the primary beam were approximately 0.5 mGy Gy at a distance of 100 mm and 0.02 mGy Gy at 300 mm from the center of the target. For the neutrons, the corresponding values of dose equivalent were found to be ~0.7 and ~0.06 mSv Gy, respectively. The measured neutron doses were comparable with the out-of-field neutron doses from a similar experiment with 20 MV x-rays, whereas photon doses for the scanning proton beam were up to three orders of magnitude lower.

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.

This work aims to generate a simple analytical model that allows estimation of peripheral photon equivalent dose to organs of individual patients, valid for any isocentric technique. Photon radiation scattered in the LINAC head has been simulated as a virtual source of radiation emitting isotropically so that, before reaching a point inside the patient, it decreases with the square law and with attenuation due to air and tissue. Leakage has been simulated as a constant background dose along the patient. Firstly, a dose-to-points basic model was proposed and parameterized by fitting it to absorbed doses measured with TLD-700 in a humanoid phantom. Secondly, this model was generalized to any other situation involving intensity-modulated beams of any size and shape. Validation of this general model, usable beyond 10 cm from the field edge, was carried out by comparing estimation with TLD-100 doses for VMAT and IMRT treatments as well as with experimental data and models existing in the bibliography. Finally, an equivalent dose-to-organs model has been proposed by rescaling individual anatomical dimensions onto a mathematical phantom in order to make an estimation of organ length for dose calculation. The parameterized extended model, accounting for intensity-modulated beams of any shape, predicts measurements with a maximum relative uncertainty of ±25%. This general model, easy to apply in a clinical routine thanks to the ready availability of input parameters, has been proposed and validated for estimation of photon equivalent doses to peripheral organs. Finally, as a first step, it has been implemented into a piece of software termed PERIPHOCAL (PERIpheral PHOton CALculation), which is easily transferred to a commercial treatment planning system (TPS).