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 2708. 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. Passingwas 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%, 3mmbut 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 significant difference was found between passing rates for multi-field and single-field optimized plans. The passing rate was 94.8% 6 0.6% for fields with range shifters and 99.0% 6 0.6% for those without (P¼ .002). Most low gamma index scores were due to steep dose gradients transverse to the measured plane. A less frequent cause of failures was an apparent systematic overestimation of the calculated dose at depths proximal to the spread-out Bragg peak. Conclusion: A comprehensive PSQA program serves to ensure the safety of a specific treatment plan and acts as a check on the entire treatment system. We propose that the 3%, 3 mm with 90% pixel passing rate is a reasonable action level for 2-dimensional comparisons of dose planes in SSPT, although more restrictive tolerance levels would be appropriate for prostate treatment plans