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In-House IMRT QA Versus External Phantom Audit Results

S Kry

S Kry*, A Molineu, P Alvarez, A Faught, J Huang, J Kerns, K Pulliam, J Tonigan, D Followill, UT MD Anderson Cancer Center, Houston, TX

TU-E-108-5 Tuesday 2:00PM - 3:50PM Room: 108

Purpose: The Radiological Physics Center (RPC) credentials institutions for clinical trials involving advanced radiotherapy techniques with anthropomorphic phantoms. This study compared phantom results with in-house IMRT QA results.

Methods: We examined RPC head phantom irradiations for which institutions had submitted their own in-house IMRT QA results for the same plan. The RPC phantom includes 6 TLD-based point dose measurements, and film for evaluating distance to agreement (DTA) between the target and a neighbouring critical structure. Treatments failed the RPC phantom if any TLD-measured dose was outside 7% or the DTA in the high gradient region between the target and critical structure was more than 4mm from the institution's TPS-calculation. Institutions' IMRT QA on the same plan was per their protocol. The institution failed IMRT QA if a) they declared that their results did not meet their internal criteria, or, b) if their point doses agreed outside 3% or planar analysis agreed with less than 90% of pixels passing a 3%/3mm (or less stringent) gamma criteria. 189 plans were examined.

Results: 23 plans failed RPC phantom irradiation, while 18 failed in-house IMRT QA. However, consistency was not observed between the two systems. Plans that failed the RPC phantom irradiation were only 22% likely to be noted as failing in-house IMRT QA (i.e., sensitivity = 0.22). Plans that passed the RPC phantom tended to pass IMRT QA; the specificity was 92%. No correlation was found between the agreement of in-house point dose and TPS versus RPC point dose and TPS.

Conclusion: Sensitivity, the most important quality of a QA process, was found to be very low overall for in-house IMRT QA as compared to the RPC phantom. These results challenge the merit of IMRT QA to detect problems in an end-to-end treatment delivery.

Funding Support, Disclosures, and Conflict of Interest: This work was supported by Public Health Service Grants CA10953 and CA081647 awarded by the National Cancer Institute, United States Department of Health and Human Services.

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