Impact of apertures on the out-of-field secondary neutron dose in collimated proton pencil-beam scanning

Objective.The advantage of apertures sharpening the edges of pencil beam scanning proton fields comes with the price of an increased out-of-field dose predominantly mediated by secondary neutrons. The objective of this study was to measure the increase in neutron ambient dose equivalent () for monoenergetic proton fields collimated by static brass apertures compared to non-collimated fields. The influence of the aperture material, i.e. brass and nickel, on the neutron contamination was investigated. The experimental results were compared with a Monte Carlo (MC) simulation framework.Approach.Experiments were conducted in a treatment room of a proton therapy center. Thewas measured with three extended-range rem-meters at multiple out-of-field positions at 2 m/1.12 m distance from the isocenter for quasi-monochromatic square proton fields (100, 140, 180 MeV) incident on a 30solid water target. The measurement results were compared to respective calculations with the TOPAS MC simulation framework.Main results.Collimating the edges of 10fields with apertures increased theon average by 23%. This increase was up to 74% in lateral direction to the beam and up to 6% in forward direction. With a reduction ofdown to 50% and 33% on average, nickel proved to be more favorable than brass. Placing a range-shifter upstream of the aperture is more favorable compared to the downstream placement since thewas on average 20% lower. Considering the associated uncertainties, the simulations and experiments agreed well with an average absolute deviation of 11.7% (1% to 34%).Significance.Concluding, the increase ofcaused by apertures sharpening scanned proton field edges is small to moderate with 23% on average. Neutron contamination can be reduced by material selection and the arrangement of beam-shaping devices. TOPAS is suited for the application under consideration.