Spot-Scanning Proton Therapy Patient- Specific Quality Assurance: Results from 309 Treatment Plans

Abstract Purpose: We report the outcomes of patient-specific quality assurance (PSQA) for spot-scanning proton therapy (SSPT) treatment plans by disease site. Patients and Methods: We analyzed quality assurance outcomes for 309 SSPT plans. The PSQA measurements consisted of 2 parts: (1) an end-to-end test in which the beam was delivered at the prescribed gantry angle and (2) dose plane measurements made from gantry angle 270°. The HPlusQ software was used for gamma analysis of the dose planes using dose-tolerance and distance-to-agreement levels of 2%, 2 mm and 3%, 3 mm, respectively. Passing was defined as a gamma score <1 in at least 90% of the pixels. Results: The overall quality assurance measurement passing rate was 96.2% for the gamma index criteria of 3%, 3 mm but fell to 85.3% when the criteria were tightened to 2%, 2 mm. The passing rate was dependent on the treatment site. With the 3%, 3 mm criteria, the passing rate was 95% for head-and-neck treatment plans and 100% for prostate plans. No signi...