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  5. Is the Standard IMRT/VMAT QA Good Enough? Investigating Log File Monte Carlo-Based Quality Assurance (LF-MC QA) as An Alternative Treatment Planning/Delivery QA Technique

Is the Standard IMRT/VMAT QA Good Enough? Investigating Log File Monte Carlo-Based Quality Assurance (LF-MC QA) as An Alternative Treatment Planning/Delivery QA Technique

C Stanhope1,4*, M Alber2,3 , M Sohn3 , D Drake4 , J Liang4 , D Yan4 , (1) Wayne State University, Royal Oak, MI, (2) Universitat Heidelberg, Tuebingen, (3) ScientificRT GmbH, Munich, (4) William Beaumont Hospital, Royal Oak, MI

Presentations

(Monday, 7/15/2019) 1:15 PM - 1:45 PM

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 Pinnacle’s collapsed-cone convolution-superposition (Plan-CS). Clinical anatomical dose was re-calculated using ScientificRT’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 (1/2/4°) 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 [-0.1±0.1%, 0.0±0.2%, -0.2±0.2%] for machine delivery error, [-3.4±1.9%, -4.6±2.8%, -1.2±2.8%] for dose calculation difference, and [0.5±2.0%, 0.2±1.2%, 2.6±4.1%] due to limited VMAT beam sampling. Drop in TCP due to calculation difference (MC-CS) was -3.1±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 97.6±2.3% [1 failure of 87.2%]. Using an action tolerance of |ΔGTV D99| < 5% and |ΔTCP| < 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.

Keywords

Monte Carlo, Quality Assurance, Treatment Verification

Taxonomy

TH- External beam- photons: Quality Assurance - IMRT

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