Neutron contribution depending on the treatment room during pediatric pencil beam scanning proton therapy

Objective.This study compares secondary neutron fields for two proton treatment rooms to identify the impact of room geometry and treatment machine design. The equivalent dose exposure to paediatric craniospinal irradiation (CSI) patients including treatment room components was further quantified.Methods.Treatment rooms in Munich and Bergen were implemented in FLUKA. 75 and 200 MeV proton beams were simulated to a PMMA phantom to investigate scoring regions at 2 m from the isocenter at 0°, 45°, 90° and 135°, with respect to the beam. Neutron spectra were collected for individual room components. Ambient dose equivalent tables were used to convert neutron fluence to dose and compare the rooms. Paediatric CSI treatment plans were simulated within the Bergen facility, and the equivalent dose to the patients was cataloged according to the component of the final neutron interaction.Results.The phantom simulations had similar total ambient dose equivalent, with values of (0.8-1.5)μSv/Gy for the 75 MeV beam and (4-25)μSv/Gy for the 200 MeV beam in scoring volumes at 0°-135°. The dose contribution from the concrete room in Bergen exceeded the Munich room (0.07±0.01versus 0.02±0.01μSv/Gy) whereas the contribution from the Munich gantry was higher (0.020±0.003versus 0.005±0.001μSv/Gy). The resulting overall neutron equivalent dose to the organs ranged (0.09-2.96) mSv/Gy for the CSI plans and (0.004-0.05) mSv/Gy for the boost plans. In certain scenarios, the patient frame, gantry and concrete room contributed up to 44% to the overall neutron equivalent dose for organs far away from the treatment fields.Significance.Inclusion of the treatment room increased the accuracy of the calculated dose (with a difference of up to 0.16 mSv/Gy) associated with personalized proton therapy, especially in organs distant from the treatment field. The relatively small differences in neutron ambient dose equivalent levels found across the two facilities suggests that the presented data can be applied where detailed room simulations are not available.