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A Sensitivity Dosimetric Study of Setup Uncertainties During Machine Commission and Annual QA

V Nguyen

V Nguyen1*, B Wang1 , C Shi2 , (1) Univ Louisville, Louisville, KY, (2) Saint Vincent Medical Center, Bridgeport, CT


PO-BPC-Exhibit Hall-12 (Saturday, March 5, 2016)  Room: Exhibit Hall

Purpose: To investigate three common potential set up uncertainties during LINAC commission & annual QA and evaluate how these set up uncertainties propagate into patient-specific QA results using gamma index analysis.
Methods: Three uncertainty scenarios were purposely introduced for gantry position tilts from 0- 3° (scenario 1), SAD changes from 100-103 cm (scenario 2), and isocenter position shifts from 0-4 mm (scenario 3). Data was simulated using a 60x60x60 cm³ solid water cube created in Varian Eclipse TPS (v.11) to replicate a 3D water tank. For each scenario, beam data profiles (crossline and diagonal) and percent depth dose were simulated individually at different field sizes and depths for three energies: 6 MV, 6MV-FFF and 10MV-FFF. A gamma analysis was used to evaluate the results using 1%/1mm criteria.
Results: Both PDDs and profiles showed larger deviations for increasing setup uncertainties as expected, while different patterns as a function of depth among the three scenarios. For example in scenario 1, a ≥90% gamma passing rate and ≤1% dose difference were observed for gantry tilted up to 2°. For 3° degree titled, gamma dropped below 90% at depth of ≥20 cm for 6MV/6FFF and depth of ≥12 cm for 10MV-FFF. This is reasonable since gantry rotations cause larger geometric variations at larger depths. However for Scenario 2, dosimetric uncertainties were larger at shallower depths. Gammas were ≤90% and dose differences were ≥1% for depth up to 20 cm for all energies. For scenario 3, a ≥90% gamma passing rate and ≤1% dose difference were seemed on ≤4mm iso-center shifted for all energies.
Conclusion: This study validated AAPM TG 142 recommendations on the mechanical and dosimetry uncertainties and provided quantitative analysis of maximum acceptance tolerances for LINAC annual QA and commission.

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