A quality assurance for respiratory gated proton irradiation with range modulation wheel

The purpose of this study was to provide periodic quality assurance (QA) methods for respiratory‐gated proton beam with a range modulation wheel (RMW) and to clarify the characteristics and long‐term stability of the respiratory‐gated proton beam. A two‐dimensional detector array and a solid water phantom were used to measure absolute dose, spread‐out Bragg peak (SOBP) width and proton range for monthly QA. SOBP width and proton range were measured using an oblique incidence beam to the lateral side of a solid water phantom and compared between with and without a gating proton beam. To measure the delay time of beam‐on/off for annual QA, we collected the beam‐on/off signals and the dose monitor‐detected pulse. We analyzed the results of monthly QA over a 15‐month period and investigated the delay time by machine signal analysis. The dose deviations at proximal, SOBP center and distal points were −0.083 ± 0.25%, 0.026 ± 0.20%, and −0.083 ± 0.35%, respectively. The maximum dose deviation between with and without respiratory gating was −0.95% at the distal point and other deviations were within ±0.5%. Proximal and SOBP center doses showed the same trend over a 15‐month period. Delay times of beam‐on/off for 200 MeV/SOBP 16 cm were 140.5 ± 0.8 ms and 22.3 ± 13.0 ms, respectively. Delay times for 160 MeV/SOBP 10 cm were 167.5 ± 15.1 ms and 19.1 ± 9.8 ms. Our beam delivery system with the RMW showed sufficient stability for respiratory‐gated proton therapy and the system did not show dependency on the energy and the respiratory wave form. The delay times of beam‐on/off were within expectations. The proposed QA methods will be useful for managing the quality of respiratory‐gated proton beams and other beam delivery systems.