Study of the responses and calibration procedures of neutron and gamma area and environmental detectors for use in proton therapy.

Ambient dose equivalent measurements with radiation protection instruments are associated to large uncertainties, mostly due to the energy dependence of the instrument response and to the dissimilarity between the spectra of the standard calibration source and the workplace field. The purpose of this work is to evaluate its impact on the performance of area and environmental detectors in the proton therapy environment, and to provide practical solutions whenever needed and possible. The study was carried out at the Centre Antoine Lacassagne (CAL) proton therapy site, and included a number of commercially available area detectors and a home-made environmental thermoluminescent dosimeter based on a polyethylene moderator loaded with TLD600H/TLD700H pairs. Monte Carlo simulations were performed with MCNP to calculate, first, missing or partially lacking instrument responses, covering the range of energies involved in proton therapy. Second, neutron and gamma spectra were computed at selected positions in and outside the CAL proton therapy bunkers. Appropriate correction factors were then derived for each detector, workplace location and calibration radionuclide source, which amounts to up to 1.9 and 1.5 for neutron and photon area detectors, respectively, and suggest that common ambient dose equivalent instruments might not meet IEC requirements. The TLD environmental system was calibrated in situ and appropriate correction factors were applied to account for the cosmic spectra. Measurements performed with this system from 2014 to 2017 around the installation were consistent with reference natural background dose data and with pre-operational levels registered at the site before the construction of the building in 1988, showing thus no contribution from the site clinical activities. An in situ verification procedure for the radiation protection instruments was also implemented in 2016 at the low energy treatment room using the QA beam reference conditions. The method presents main methodological, practical and economic advantages over external verifications.

Theoretical dosimetric evaluation of carbon and oxygen minibeam radiation therapy.

PURPOSE: Charged particles have several advantages over x-ray radiations, both in terms of physics and radiobiology. The combination of these advantages with those of minibeam radiation therapy (MBRT) could help enhancing the therapeutic index for some cancers with poor prognosis. Among the different ions explored for therapy, carbon ions are considered to provide the optimum physical and biological characteristics. Oxygen could be advantageous due to a reduced oxygen enhancement ratio along with a still moderate biological entrance dose. The aforementioned reasons justified an in-depth evaluation of the dosimetric features of carbon and oxygen minibeam radiation therapy to establish the interest of further explorations of this avenue. MATERIALS AND METHODS: The GATE/Geant4 6.2 Monte Carlo simulation platform was employed to simulate arrays of rectangular carbon and oxygen minibeams (600 µm × 2 cm) at a water phantom entrance. They were assumed to be generated by means of a magnetic focusing. The irradiations were performed with a 2-cm-long spread-out Bragg peak (SOBP) centered at 7-cm-depth. Several center-to-center (c-t-c) distances were considered. Peak and valley doses, as well as peak-to-valley dose ratio (PVDR) and the relative contribution of nuclear fragments and electromagnetic processes were assessed. In addition, the type and proportion of the secondary nuclear fragments were evaluated in both peak and valley regions. RESULTS: Carbon and oxygen MBRT lead to very similar dose distributions. No significant advantage of oxygen over carbon ions was observed from physical point of view. Favorable dosimetric features were observed for both ions. Thanks to the reduced lateral scattering, the standard shape of the depth dose curves (in the peaks) is maintained even for submillimetric beam sizes. When a narrow c-t-c is considered (910-980 µm), a (quasi) homogenization of the dose can be obtained at the target, while a spatial fractionation of the dose is maintained in the proximal normal tissues with low PVDR. In contrast when a larger c-t-c is used (3500 µm) extremely high PVDR (≥ 50) are obtained in normal tissues, corresponding to very low valley doses. This suggests that carbon and oxygen MBRT might lead to a significant reduction of normal tissue complication probability. The main participant to the valley doses are secondary nuclear products at all depths. Among them the highest yield in normal tissues corresponds to the lightest fragments, neutrons and protons. Heavier fragments are dominant in the valleys only at the target position, which might favor tumor control. CONCLUSIONS: The computed dose distributions suggest that a spatial fractionation of the dose combined to the use of submillimetric field sizes might allow profiting from the high efficiency of carbon and oxygen ions for the treatment of radioresistant tumors, while preserving normal tissues. Only biological experiments could confirm the shifting of the normal tissue complication probability curves. The authors' results support the further exploration of this avenue.

Optimization of the mechanical collimation for minibeam generation in proton minibeam radiation therapy.

PURPOSE: The dose tolerances of normal tissues continue to be the main barrier in radiation therapy. To lower it, a novel concept based on a combination of proton therapy and the use of arrays of parallel and thin beams has been recently proposed: proton minibeam radiation therapy (pMBRT). It allies the inherent advantages of protons with the remarkable normal tissue preservation observed when irradiated with submillimetric spatially fractionated beams. Due to multiple Coulomb scattering, the tumor receives a homogeneous dose distribution, while normal tissues in the beam path benefit from the spatial fractionation of the dose. This promising technique has already been implemented at a clinical center (Proton therapy Center of Orsay) by means of a first prototype of a multislit collimator. The main goal of this work was to optimize the minibeam generation by means of a mechanical collimation. METHODS: Monte Carlo simulations (GATE V7.1) were used to evaluate the influence of the collimator material (brass, nickel, iron, tungsten), thickness, phantom-to-collimator distance (PCD), among other parameters, on the dose distributions. Maximization of the peak-to-valley dose ratios (PVDR) in normal tissues along with minimization of full width at half maximum, penumbras and neutron contamination were used as figures of merit. As a starting point for the optimization, the collimator employed in our previous works was used. It consisted in 400 µm × 2 cm slits with a center-to-center distance (c-t-c) of 3200 µm. As the main targets of pMBRT will be neurological cases, 100 MeV energy proton minibeams were considered. This energy range would allow treating tumors located at the center of the brain (the worst scenario). RESULTS: Tungsten and brass are the most advantageous materials among those considered. A tungsten collimator provides the highest PVDR and lowest penumbra. Although the neutron yield generated in the tungsten collimator is 3 times higher than that of the other materials, the biologic neutron doses at the patient position amount to less than 0.05% and 0.7% of the peak and valley doses, respectively. In addition, shorter PCD than the one currently used (7 cm) leads to thinner beams (enhancing the dose-volume effects), accompanied, however, by an increase of neutron dose at the phantom surface. Finally, no gain in dose distributions is obtained by using nonparallel slits. CONCLUSIONS: The collimator design and irradiation configuration have been optimized to minimize the angular spread, deliver the highest PVDR and the lowest valley possible in the normal tissues in pMBRT. We have also confirmed that even though the neutron yield generated in the multislit collimator is higher with respect to the one produced by the collimators used in conventional proton therapy, the increase of biological neutron dose in the patient will remain low (less than 1%).