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Secondary Neutrons From Clinical Electron Beams

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G Al Makdessi

G Al Makdessi1*, J Kildea2 , A Licea3 , (1) Medical Physics Unit, McGill University Health Centre, Montreal, Quebec, (2) Medical Physics Unit, McGill University Health Centre, Montreal, Quebec, (3) Canadian Nuclear Safety Commission, Ottawa, Ontario


SU-I-GPD-T-235 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: As is the case for high-energy photon beam therapy (> 10 MV), secondary neutrons are produced during electron therapy. These neutrons 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.

Methods: Neutron spectra were measured for secondary neutrons arising from a clinical electron beam of a Varian linear accelerator with maximum electron energy of 20 MeV. Measurements were performed using the Nested Neutron Spectrometer (NNS, Detec Inc, Gatineau Quebec) [1]. 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 raw neutron data were unfolded using the Maximum a Posteriori (MAP) method [2] (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) [3] that penalizes any high noise component. Additionally, the ICRU conversion coefficients were used to calculate the neutron dose equivalent rates for the electron beams.

Results: The effect of several parameters on the neutron fluence rate and on the neutron dose equivalent rate were studied: (1) As the energy of the electron beam increases, the neutron fluence rate and dose equivalent rate increase. (2) The presence of a solid water phantom on the couch does not affect the neutron spectra. (3) When closing the jaws in the treatment head, the neutron fluence rate and dose equivalent rate increase significantly.

Conclusion: In this work, the generation of secondary neutrons around clinical electron beams was investigated. The main source of these secondary neutrons appears to lie in the treatment head with only a small in-phantom component.

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