This study aimed to report the outcomes of patient-specific quality assurance (QA) in spot-scanning proton therapy using a two-dimensional ionisation chamber array and investigate the relationship between gamma passing rate and plan parameters.

Patient-specific QA was performed and evaluated by gamma analysis using a 3% dose difference and 2-mm distance-to-agreement with 172 treatment plans in the head and neck, breast, chest, abdominal and pelvic regions. The outcomes of patient-specific quality assurance regarding the gamma passing rate of the treatment sites, monitor unit (MU) per spot, measurement depth, range shifter, number of spots, energy layer and target volume were analysed.

No significant difference (p = 0·10) in the gamma passing rates between the treatment sites. The gamma passing rate was >98% in all the regions. The overall result of patient-specific QA with the gamma evaluation was 99·1 ± 1·6%. For the MU per spot, range shifter and measurement depth, the gamma passing rate was >98%. The gamma passing rate of the number of spots, energy layer and target volume was >97%.

Patient-specific QA measurements showed that the gamma passing rate was >98% and was independent of the treatment site, MU per spot, range shifter, number of spots, energy layer and target volume but depend on measurement depth (p < 0·05). A gamma index of 3%, 2 mm forms reasonable criteria for patient-specific QA in spot-scanning proton therapy.