Room: Exhibit Hall | Forum 4
Purpose: A treatment planning/delivery QA tool using linac log files (LF) and Monte Carlo (MC) dose calculation was investigated as a standalone alternative to ArcCHECK
Methods: LF-MC QA was carried out for 42 clinical VMAT patients planned and optimized using Pinnacleâ€™s collapsed-cone convolution-superposition (Plan-CS). Clinical anatomical dose was re-calculated using ScientificRTâ€™s SciMoCa Monte Carlo (Plan-MC) and dose discrepancy due to the calculation algorithm was isolated. Comparing LF-MC to Plan-MC, discrepancy due to delivery error was isolated. The effect of varying control point spacing (1/2/4Â°) on CS dose was evaluated. Dose discrepancies were evaluated using PTV Dmean/D99/D1 and tumor control probability (TCP). Dose discrepancy due to calculation algorithm was further assessed as a function of heterogeneity and beam modulation complexity (MU/Rx). Heterogeneity effects were assessed for 5 lung and 5 H&N cases by overriding the patient anatomies to water. LF-MC QA results were compared to clinical ArcCHECK QA results. Various LF-MC QA pass/fail protocols were assessed.
Results: Percent differences in [PTV Dmean, D99, D1] were [-0.1Â±0.1%, 0.0Â±0.2%, -0.2Â±0.2%] for machine delivery error, [-3.4Â±1.9%, -4.6Â±2.8%, -1.2Â±2.8%] for dose calculation difference, and [0.5Â±2.0%, 0.2Â±1.2%, 2.6Â±4.1%] due to limited VMAT beam sampling. Drop in TCP due to calculation difference (MC-CS) was -3.1Â±1.8% [min -5.7%]. 41% of PTV D99 dose calculation difference was due to beam complexity. Heterogeneity effects were negligible for H&N. For lung, 18% of dose calculation difference on PTV Dmean was due to heterogeneity. Despite substantial LF-MC QA dose discrepancies, 3%/2mm per-beam ArcCHECK pass rates were 97.6Â±2.3% [1 failure of 87.2%]. Using an action tolerance of |Î”GTV D99| < 5% and |Î”TCP| < 4%, 6/14 complex (MU/Rx>3.5) and 2/28 simple (MU/Rx < 3.5) plans were deemed failing.
Conclusion: ArcCHECK QA was consistently incapable of catching clinically relevant dose discrepancies as calculated on the patient anatomy using LF-MC QA.
Funding Support, Disclosures, and Conflict of Interest: Partially funded by an Elekta research grant.