(Wednesday, 7/17/2019) 10:30 AM - 11:00 AM
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 γ-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: ΔD(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 γ-test (97.7 +/- 3.2%) and a 3%/1mm ΔD(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 γ-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.
Keywords
Protons, Quality Assurance, Setup Errors
Taxonomy
TH- External Beam- Particle therapy: Proton therapy - quality assurance
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