Abstract
Neutrons are the main type of secondary particles emitted in proton-therapy. Because of the risk of secondary cancer and other late occurring effects, the neutron dose should be included in the out-of-field dose calculations. A neutron spectrometer has to be used to take into account the energy dependence of the neutron radiological weighting factor. Due to its high dependence on various parameters of the irradiation (beam, accelerator, patient), the neutron spectrum should be measured independently for each treatment. The current reference method for the measurement of the neutron energy, the Bonner Sphere System, consists of several homogeneous polyethylene spheres with increasing diameters equipped with a proportional counter. It provides a highresolution reconstruction of the neutron spectrum but requires a time-consuming work of signal deconvolution. New neutron spectrometers are being developed, but the main experimental limitation remains the high neutron flux in proton therapy treatment rooms. A new model of a real-time neutron spectrometer, based on a Recoil Proton Telescope technology, has been developed at the IPHC. It enables a real-time high-rate reconstruction of the neutron spectrum from the measurement of the recoil proton trajectory and energy. A new fast-readout microelectronic integrated sensor, called FastPixN, has been developed for this specific purpose.A first prototype, able to detect neutrons between 5 and 20 MeV, has already been validated for metrology with the AMANDE facility at Cadarache. The geometry of the new Recoil Proton Telescope has been optimized via extensive Geant4 Monte Carlo simulations. Uncertainty sources have been carefully studied in order to improve simultaneously efficiency and energy resolution, and solutions have been found to suppress the various expected backgrounds. We are currently upgrading the prototype for secondary neutron detection in proton therapy applications.
Highlights
Secondary neutrons in proton-therapyDuring a proton-therapy treatment, a lot of secondary neutrons are produced by the interaction of the proton beam with the patient (internal neutrons), and with the accelerator (external neutrons) for some treatments, via AZ X(p, n)AZ+1Y reactions
As the biological effect of neutrons is dependent on their energy [2], a neutron spectrometry is mandatory for a precise estimation of the neutron equivalent dose
The fast neutrons being the greatest contributors to the neutron dose, our approach is to focus on this energy range while other systems are developed for measuring thermal neutrons
Summary
During a proton-therapy treatment, a lot of secondary neutrons are produced by the interaction of the proton beam with the patient (internal neutrons), and with the accelerator (external neutrons) for some treatments, via AZ X(p, n)AZ+1Y reactions. Due to their high radiological weighting factor, the neutron dose should be monitored. The angle to the proton beam, which changes the shape of the fast neutron spectrum [5][6][7][8][9]. Scarce experimental data coupled to extrapolation via Monte Carlo simulation is currently used to estimate the dose, but this leads to large discrepancies between the simulation and experimental data
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