G DeFillippo* S Mashouf, P Teo, J Foy, G Khelashvili, P Yadav, M Gopalakrishnan*, I Das, Northwestern University Feinberg School of Medicine, Chicago, IL

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  M Gopalakrishnan

PO-GePV-T-223 (Sunday, 7/10/2022)   [Eastern Time (GMT-4)]

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Purpose: To derive an equivalent set of criteria to be used clinically in gradient compensation (GC) analysis versus TG-218 gamma analysis for patient-specific QA (PSQA) in modulated therapy.

Methods: We compared 3D gradient compensation (GC) analysis at different criteria with 3D gamma analysis for a cohort of 123 patient treatment plans (59% pelvis, 41% head and neck) on the SNC Patient program (Sun Nuclear). The GC method utilized a 20% threshold and non-global normalization with varying levels of dose difference (%) and distance-to-agreement (mm) values. The 3D gamma analysis utilized the TG-218 standard of 10% threshold, 3% dose difference and 2mm distance-to-agreement with global analysis. Additionally, all measurements were performed on the ArcCHECK (Sun Nuclear) device with an absolute dose calibration factor measured on site.

Results: 124 patient plans (with 1 erroneous outlier dropped) were analyzed at the following criteria: Gradient compensation, 20% threshold, absolute dose, local normalization: 1%/1mm, 2%/2mm, 3%/3mm, 5%/5mm; Gamma analysis, 10% threshold, absolute dose, global normalization: 3%/2mm. The PSQA pass rate average differed by less than 1% between sites, with 88% differing by less than 5%. GC at 2%/2mm criteria (GC22) gave the closest results to the TG-218 standard with average pass rate of 92.2% (± ơ=4.5%) versus 92.1% (± ơ=4.5%) respectively and absolute difference |GC22-TG218| of 2.8% (± ơ = 3.6%) among all sites. 74% of patients passed at 90% or higher at GC22 criteria, while 70% of patients passed at 90% or higher with TG-218 standards.

Conclusion: 3D gradient compensation analysis at 2%/2mm, 20% threshold and no global normalization criteria is an acceptable substitute for TG-218 standards for quality assurance of IMRT/VMAT patients in clinical practice.

Keywords

Quality Assurance

Taxonomy

TH- External Beam- Photons: Quality Assurance - IMRT/VMAT

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