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Clinical Impact of Minor Errors Discovered in Conventional IMRT QA On Patient Treatment

S Chang

S Chang1*, M Lawrence1, T Cullip1, S Wang1, D Fried2, (1) UNC School of Medicine, Chapel Hill, NC, (2) UTH-GSBS, Houston, TX

SU-E-T-166 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To demonstrate that conventional IMRT QA metrics and passing criteria should be replaced by DVH-based criteria that can efficiently interpret the clinical impact of minor errors found for head and neck and prostate cancer patient treatments.

Methods: 41 prostate cancer and 31 head & neck (H&N) cancer clinical IMRT QAs using MapCheck were retrospectively analyzed using three different tightness of the pass/fail metrics (3%/3mm, 5%/4mm, and 7%/5mm). An in-house TPS, PLanUNC, was used to reproduce the accelerator-delivered plan from the beams measured by MapCheck in IMRT QA. The impact of the deviation/error discovered in the per-field IMRT QA on PTV D95, mean dose and max. dose of major OARs are computed and correlated with the conventional IMRT QA pass rate and tightness of IMRT metrics, and compared with the impact of a 2mm random setup error on patient dosimetry.

Results: Statistically there is no correlation between the quality of DVH-based dosimetry for PTV and OARs and the conventional IMRT QA metrics and the criteria tightness. The standard deviation of errors in PTV D95 for H&N and prostate treatments due to a 2mm random setup error are 0.7% and 2.2%, respectively. The same standard deviation due to errors discovered in IMRT QA is 1.1% and 1.0% for H&N and prostate, respectively. Our data shows that the clinical impact of a given error found in conventional IMRT QA is treatment site dependent. Based on this study, reducing IMRT errors found in H&N patients may have more clinical impact than for prostate patients.

Conclusion: While conventional per-field IMRT QA metrics are effective in catching gross error for patient safety it fails to interpret their clinical impact. A DVH-based IMRT QA evaluation should be used instead to timely and effectively identify and reduce the minor IMRT QA errors that have clinical significance.

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