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.

The purpose of this study is to evaluate variation in the treatment hold pattern and quantify its dosimetric impact in breath-hold radiotherapy, using fraction-specific post-treatment quality assurance.

A patient with lung mets treated using intensity-modulated radiation therapy (IMRT) with active breath coordinator (ABC) was recruited for the study. Treatment beam hold conditions were recorded for all the 25 fractions. The linearity and reproducibility of the dosimetric system were measured. Variation in the dose output of unmodulated open beam with beam hold was studied. Patient-specific quality assurance (PSQA) was performed with and without beam hold, and the results were compared to quantify the dosimetric impact of beam hold.

There was a considerable amount of variation observed in the number of beam hold for the given field and the monitor unit at which the beam held. Linearity and reproducibility of the dosimetric system were found within the acceptable limits. The average difference over the 25 measurements was 0·044% (0·557 to −0·318%) with standard deviation of 0·248.

Patient comfort with the ABC system and responsiveness to the therapist communication help to maintain consistent breathing pattern, in turn consistent treatment delivery pattern. However, the magnitude of dosimetric error is much less than the acceptable limits recommended by IROC. The dosimetric error induced by the beam hold is over and above the dose difference observed in conventional PSQA.