2276 - Experiments and Monte Carlo Simulations on Out-of-Field Secondary Neutron Dose in Proton Pencil Beam Scanning with Collimating Apertures

J. N. Esser¹, and C. Bäumer^2,3; ¹West German Proton Therapy Centre Essen, Essen, Germany, ²Department of Physics, TU Dortmund University, Dortmund, Germany, ³West German Proton Therapy Centre Essen (WPE), Essen, Germany

Purpose/Objective(s): By employing a field-shaping aperture in collimated proton pencil-beam scanning, the lateral dose fall-off (penumbra) can be significantly reduced while maintaining the same in-field dose coverage. Thus, the normal tissue surrounding the target volume can be spared more effectively. However, the secondary neutron contamination inside proton treatment rooms originates mainly from proton interactions in field-shaping devices and the patient itself. It causes increased doses in the out-of-field volume of the patient associated with risk of late effects. This study investigated the impact of the additional use of a collimating aperture on the out-of-field dose and if the clinical advantage still persists despite the expected increase in neutron dose. Materials/

Methods: Monte Carlo (MC) simulations were used to reproduce corresponding measurements conducted inside a treatment room of a clinical proton therapy facility. The respective MC code had previously been validated for this purpose. Multiple commercial neutron detectors measured the ambient dose equivalent at various out-of-field positions around a cubic solid water phantom, irradiated with a number of mono-energetic fields with and without an aperture. An elaborate treatment room model has been developed for the simulations, considering the most important structures and respective materials regarding secondary neutron production.
Results: There was a reasonable agreement of simulation and measurement results within 1 to 38%. The evaluation of the impact of an aperture on the out-of-field dose distribution revealed that the neutron ambient dose equivalent may increase by up to a factor of two at larger neutron emission angles with respect to the beam direction (90°) compared to the case without the aperture. In beam direction (0°), no significant increase in neutron dose was observed. Moreover, an additional material benchmark involving two full metal disks revealed that the secondary neutron production is considerably higher in a brass aperture compared to nickel, yielding dose values up to twice as high.
Conclusion: The observed increase in out-of-field neutron dose can be traced back to the production of evaporation neutrons in the aperture material by collimated protons. In direct comparison, the increased neutron dose does not outweigh the clinical advantage of the dose reduction achieved with a sharper penumbra considering the difference in orders of magnitude of the respective out-of-field doses. However, since the out-of-field neutron dose affects the whole body of the patient even far outside the treated volume and the associated risk of late effects is not yet reliably assessed, any significant increase in neutron dose should be considered and avoided whenever possible. Hence, the results of this work also suggest the use of nickel over brass as a material for collimating apertures.