Monte Carlo study of neutron spectra unfolding for a proton beam using MLEM

Secondary neutrons are generated during proton therapy treatments, being a source of stray dose to the patient and personnel. Since the dose deposited by neutrons is directly related to their energy, determining their energy spectrum is crucial for the correct assessment of neutron doses. The aim of this work is to use Monte Carlo (MC) simulations to reproduce the generation of secondary stray neutrons in a proton therapy accelerator head and deconvolute their fluence energy spectrum using a Maximum Likelihood Expectation Maximization (MLEM) algorithm. This unfolding code utilizes a detector response to deconvolute spectra. Therefore, the response of a newly designed Extended Bonner Sphere Spectrometer (EBSS) mesh model, capable of detecting high-energy neutrons, has been evaluated by means of MC simulations. To deconvolute the neutron fluence energy spectrum with MLEM, the EBSS has been placed at 30 and 115 cm distance from the accelerator head in the direction of the proton beamline, and the secondary neutron fluence has been calculated in the volume of the detector. These measures served to unfold the MC simulated neutron fluence spectrum with absence of treatment room elements, by using the self-developed MLEM algorithm. The results indicate that a MLEM algorithm and MC simulations can be used as tools to assess neutron spectra in radiation therapies using protons.