Neutron Radiation Dose Measurements in a Scanning Proton Therapy Room: Can Parents Remain Near Their Children During Treatment?

Purpose: This study aims to characterize the neutron radiation field inside a scanning proton therapy treatment room including the impact of different pediatric patient sizes. Materials and Methods: Working Group 9 of the European Radiation Dosimetry Group (EURADOS) has performed a comprehensive measurement campaign to measure neutron ambient dose equivalent, H*(10), at eight different positions around 1-, 5-, and 10-year-old pediatric anthropomorphic phantoms irradiated with a simulated brain tumor treatment. Several active detector systems were used. Results: The neutron dose mapping within the gantry room showed that H*(10) values significantly decreased with distance and angular deviation with respect to the beam axis. A maximum value of about 19.5 µSv/Gy was measured along the beam axis at 1 m from the isocenter for a 10-year-old pediatric phantom at 270° gantry angle. A minimum value of 0.1 µSv/Gy was measured at a distance of 2.25 m perpendicular to the beam axis for a 1-year-old pediatric phantom at 140° gantry angle.The H*(10) dependence on the size of the pediatric patient was observed. At 270° gantry position, the measured neutron H*(10) values for the 10-year-old pediatric phantom were up to 20% higher than those measured for the 5-year-old and up to 410% higher than for the 1-year-old phantom, respectively. Conclusions: Using active neutron detectors, secondary neutron mapping was performed to characterize the neutron field generated during proton therapy of pediatric patients. It is shown that the neutron ambient dose equivalent H*(10) significantly decreases with distance and angle with respect to the beam axis. It is reported that the total neutron exposure of a person staying at a position perpendicular to the beam axis at a distance greater than 2 m from the isocenter remains well below the dose limit of 1 mSv per year for the general public (recommended by the International Commission on Radiological Protection) during the entire treatment course with a target dose of up to 60 Gy. This comprehensive analysis is key for general neutron shielding issues, for example, the safe operation of anesthetic equipment. However, it also enables the evaluation of whether it is safe for parents to remain near their children during treatment to bring them comfort. Currently, radiation protection protocols prohibit the occupancy of the treatment room during beam delivery.

A comprehensive Monte Carlo study of out-of-field secondary neutron spectra in a scanned-beam proton therapy gantry room.

PURPOSE: To simulate secondary neutron radiation fields that had been measured at different relative positions during phantom irradiation inside a scanning proton therapy gantry treatment room. Further, to identify origin, energy distribution, and angular emission of the secondary neutrons as a function of proton beam energy. METHODS: The FLUKA Monte Carlo code was used to model the relevant parts of the treatment room in a scanned pencil beam proton therapy gantry including shielding walls, floor, major metallic gantry-components, patient table, and a homogeneous PMMA target. The proton beams were modeled based on experimental beam ranges in water and spot shapes in air. Neutron energy spectra were simulated at 0°, 45°, 90° and 135° relative to the beam axis at 2m distance from isocenter for monoenergetic 11×11cm2 fields from 200MeV, 140MeV, 75MeV initial proton beams, as well as for 118MeV protons with a 5cm thick PMMA range shifter. The total neutron spectra were scored for these four positions and proton energies. FLUKA neutron spectra simulations were crosschecked with Geant4 simulations using initial proton beam properties from FLUKA-generated phase spaces. Additionally, the room-components generating secondary neutrons in the room and their contributions to the total spectrum were identified and quantified. RESULTS: FLUKA and Geant4 simulated neutron spectra showed good general agreement with published measurements in the whole simulated neutron energy range of 10-10 to 103MeV. As in previous studies, high-energy (E≥19.6MeV) neutrons from the phantom are most prevalent along 0°, while thermalized (1meV≤E<0.4eV) and fast (100keV≤E<19.4MeV) neutrons dominate the spectra in the lateral and backscatter direction. The iron of the large bending magnet and its counterweight mounted on the gantry were identified as the most determinant sources of secondary fast-neutrons, which have been lacking in simplified room simulations. CONCLUSIONS: The results helped disentangle the origin of secondary neutrons and their dominant contributions and were strengthened by the fact that a cross comparison was made using two independent Monte Carlo codes. The complexity of such room model can in future be limited using the result. They may further be generalized in that they can be used for an assessment of neutron fields, possibly even at facilities where detailed neutron measurements and simulations cannot be performed. They may also help to design future proton therapy facilities and to reduce unwanted radiation doses from secondary neutrons to patients.

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

BACKGROUND AND PURPOSE: Systematic investigation of the energy and angular dependence of secondary neutron fluence energy distributions and ambient dose equivalents values (H*(10)) inside a pencil beam scanning proton therapy treatment room using a gantry. MATERIALS AND METHODS: Neutron fluence energy distributions were measured with an extended-range Bonner sphere spectrometer featuring ³He proportional counters, at four positions at 0°, 45°, 90°, and 135° with respect to beam direction and at a distance of 2 m from the isocenter. The energy distribution of secondary neutrons was investigated for initial proton beam energies of 75 MeV, 140 MeV, and 200 MeV, respectively, using a 2D scanned irradiation field of 11 × 11 cm² delivered to a 30 × 30 × 30 cm³ PMMA phantom. Additional measurements were performed at a proton energy of 118 MeV including a 5 cm range-shifter (PMMA), yielding a Bragg peak position similar to that of 75 MeV protons. RESULTS: Ambient dose equivalent values from 0.3 µSv/Gy (75 MeV; 90°) to 24 µSv/Gy (200 MeV; 0°) were measured inside the treatment room at a distance of 2 m from the isocenter. H*(10) values were lower (by factors of up to 7.2 (at 45°)) at 75 MeV compared to those at 118 MeV with the 5 cm range-shifter. At 0° and 45°, an evaporation peak was found in the measured neutron fluence energy distributions, at neutron energies around MeV, which contributes about 50% to total H*(10) values, for all investigated proton beam energies. CONCLUSIONS: This study showed a pronounced increase of secondary neutron H*(10) values inside the proton treatment room with increasing proton energy without beam modifiers. For example, in beam direction this increase was about a factor of 50 when protons of 75 MeV and 200 MeV were compared. The existence of a peak of secondary neutrons in the MeV region was demonstrated in beam direction (0°). This peak is due to evaporation neutrons produced in the existing surrounding materials such as those used for the gantry. Therefore, any simulation of the secondary neutrons within a proton treatment room must take these materials into account. In addition, the results obtained here show that the use of a range-shifter increases the production of secondary neutrons inside the treatment room. Using a range-shifter, the higher neutron doses observed mainly result from the higher incident proton energy (118 MeV instead of 75 MeV when no range-shifter was used), due to higher neutron production cross-sections.

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

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.

An unattended device for high-voltage sampling and passive measurement of thoron decay products.

An integrating measurement device for the concentration of airborne thoron decay products was designed and calibrated. It is suitable for unattended use over up to several months also in inhabited dwellings. The device consists of a hemispheric capacitor with a wire mesh as the outer electrode on ground potential and the sampling substrates as the inner electrode on +7.0 kV. Negatively charged and neutral thoron decay products are accelerated to and deposited on the sampling substrates. As sampling substrates, CR39 solid-state nuclear track detectors are used in order to record the alpha decay of the sampled decay products. Nuclide discrimination is achieved by covering the detectors with aluminum foil of different thickness, which are penetrated only by alpha particles with sufficient energy. Devices of this type were calibrated against working level monitors in a thoron experimental house. The sensitivity was measured as 9.2 tracks per Bq/m(3) × d of thoron decay products. The devices were used over 8 weeks in several houses built of earthen material in southern Germany, where equilibrium equivalent concentrations of 1.4-9.9 Bq/m(3) of thoron decay products were measured.