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 (H ∗ n (10) ) for monoenergetic square 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 contamnation was investigated. The experimental results were compared to a Monte Carlo (MC) simulation framework. Approach: Experiments were conducted in a treatment room of a clinical proton therapy center. The H ∗ n (10) was 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 30×30×30 cm 3 solid water target. The measurement results were compared to the respective calculations with the TOPAS MC simulation framework. Main results: Collimating the edges of 10 × 10 cm2 fields with apertures increased the H ∗ n (10) on average by 23%. This increase was up to 74% in lateral direction to the initial beam and up to 6% in forward direction. With a reduction of H ∗ n (10) down 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 the H ∗ n (10) was on average 20% lower. Considering the associated uncertainties, the simulations and experiments agreed well with an average deviation of 11.7% (1% to 34%). Significance: Concluding, the increase of H ∗ n (10) caused 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.