Secondary neutrons around clinical electron and proton beams

As is the case for high-energy photon beam therapy (> 10 MV), secondary neutrons are produced during electron and proton therapy. These neutrons, which extend in energy up to the maximum energy of the primary beam, result in a whole-body radiation dose to the patient that presents a risk for a second, radiation-induced, cancer. To account for the second-cancer risk, it is necessary to understand the spectra of the neutrons involved.In this research project, the spectra of secondary neutrons arising from clinical electron and proton beams were measured. The neutron spectra around electron beams were measured for a Varian linear accelerator with maximum electron energy of 20 MeV. The neutron spectra around proton beams were measured in a treatment room with a gantry and a dedicated nozzle at the Skandion proton therapy clinic in Uppsala, Sweden. The proton beams were generated by the compact IBA Proteus Plus cyclotron, which has a maximum proton energy of 230 MeV.Measurements were performed using the Nested Neutron Spectrometer (NNS™, Detec Inc, Gatineau Quebec). The NNS™ comprises a central He-3 detector (active neutron detection) with seven high density polyethylene shells that may be arranged around the detector in Russian doll fashion. The neutron spectra arising from proton beams were measured using a modified version of the NNS™ incorporating a brass moderator shell to provide an extended energy range (EER) for the detection of high-energy neutrons. The raw neutron data were unfolded using the Maximum a Posteriori (MAP) method (custom-written in C++) and the vendor-supplied moderator response functions for the NNS™. Note that the MAP is a modified version of the Maximum Likelihood Estimation Method (MLEM) that penalizes any high noise component. To plot the secondary neutron fluence as a function of energy, the ROOT data analysis library from CERN was used. Additionally, the ICRP conversion coefficients were used to calculate the neutron ambient dose equivalent rates around the electron and proton beams.The effect of several parameters on the neutron fluence rate and on the neutron dose equivalent rate were studied for each primary beam situation. For the electron beams, it was observed that as the energy of the primary electron beam increases, the neutron fluence rate and dose equivalent rate increase. In addition, the presence of a solid water phantom in the primary electron beam does not affect the neutron spectra. However, when closing the jaws in the treatment head, the neutron fluence rate and dose equivalent rate increase significantly which means that the main production of secondary neutrons around electron beams lies in the treatment head of the linear accelerator.For the proton beams, it was observed that as the energy of the primary proton beam increases, the neutron fluence rate and dose equivalent rate increase. The presence of a water phantom in the primary beam significantly enhances the direct and evaporation peaks of the neutron spectra which indicates that an important production of secondary neutrons around proton beams happens in the patient body. The evaporation and direct peaks of the neutron spectrum dominate the thermal peak when measuring on the couch (close to the neutron source), while the thermal peak dominates when measuring in the maze-room junction (far from the neutron source).

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