Room: Exhibit Hall | Forum 7
Purpose: PSQA measurement analysis depends on generating metrics representative of agreement between calculated and measured dose planes. This work introduces a dose plane comparison algorithm, based on a cylindrical search shape (as opposed to ellipse-based Î³-test), with search limits empirically determined from machine QA. Dose plane agreement is reported at each measured pixel as the dose difference minimum (DDM) within an empirically-established search radius: Î”D(r).
Methods: Scanning-Beam Proton beamlet accuracy was sampled over a 6-month period across the full deliverable field dimensions to determine a weighted radial positional accuracy at a 99.5% confidence level. DDM analysis was performed for 75 patients. Image-registration shifts were tracked to monitor set-up error and to isolate the search plane radius to beamlet deviation. Pass rate was assigned as the percentage of measurements where <3% dose agreement was found within the determined search limit.
Results: >99.5% of radial beamlet deviations were less than 1.0mm. The pass rate (mean +/- std. dev) saw no change between a 3%/2mm Î³-test (97.7 +/- 3.2%) and a 3%/1mm Î”D(r) (97.6 +/- 3.2%).
Conclusion: Spot-delivery spatial accuracy was well within 1mm based on extensive QA and delivery logs. However, the elliptically-shaped bound of the Î³-test is too exclusive at 1-mm. The cylindrical search shape of the new algorithm accepts all pixels with <3% agreement inside the search area, proposed herein as more relevant to plan quality, and also provides additional diagnostic information by reporting dose deviation magnitude per pixel.