Secondary neutron ambient dose equivalent measurement of the wobbling system of a proton beam radiotherapy facility

Purpose The purpose of this study was to assess the neutron ambient dose equivalent (H*(10)) per prescribed proton dose (D), H*(10)/D, for a wobbling system of the first proton beam facility in Taiwan and compare the H*(10)/D values with those of other facilities, including two wobbling system, two double scattering system, and one uniform scanning proton facilities. Materials and methods A wide energy neutron detector, WENDI-II, was used to measure the H*(10) at a distance, d, of 50–225 cm between the center of WENDI-II and the isocenter. The effects of proton beam range and SOBP width were assessed for a middle wobbling diameter. Thereafter, the H*(10)/D values were compared with those of the uniform scanning proton beam facility. The H*(10)/D values were measured at a d of 50–225 cm by using beam energy of 230 and 150 MeV as well as an SOBP width of 6 and 10 cm. The H*(10)/D values of the maximum energy and 150 MeV of the proton beam facilities were compared. Results The measured H*(10)/D values were a function of a proton beam range for SOBP widths of 5 and 10 cm. The H*(10)/D values ranged from 0.2 to 2.3 mSv/Gy at a range of 50–269 mm for 5-cm SOBP beams and from 0.2 to 2.9 mSv/Gy for 10-cm SOBP beams. The H*(10)/D values for 230-MeV proton beams increased by 20.4% when the width of SOBP was increased from 6 to 10 cm. These values for 230 MeV at a d of 50 cm were four times those of 150 MeV. The H*(10)/D values increased with the beam scanning area. Moreover, the H*(10)/D values of the medium and large wobbling diameters were respectively 2.1 and 3.3 times those of the small wobbling diameter. Conclusion We assessed the H*(10)/D values of the wobbling system of the first proton beam radiotherapy facility in Taiwan. The H*(10)/D values increased as the wobbling size increased, and these values decreased with increasing distance between the measurement position and isocenter. The H*(10)/D values of the wobbling nozzle proton beam system were higher than those of the uniform scanning system because of the interaction between proton beams and the internal components of the nozzle. The discrepancies in H*(10)/D values between facilities depend on the design of a proton beam line and the operational setup of the irradiated condition. Our experiment can serve as a reference for comparing secondary neutron radiation in wobbling system of the proton beam system.