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Calculation-Based Patient Specific IMRT QA Detects Potential Errors That Measurement-Based QA Does Not

S Mossahebi

S Mossahebi*, K Langen , M Guerrero , B Yi , W Lu , C Kalavagunta , K Prado , S Chen , University of Maryland School of Medicine, Baltimore, MD


SU-F-T-286 (Sunday, July 31, 2016) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Calculation-based patient-specific IMRT/VMAT QA, as a DICOM-RT-based plan verification system, has been commissioned and is currently being used at our institution. We report our investigation of IMRT/VMAT plan verification failures/warnings out of more than 1,000 plans verified over an 18-month period.

Methods: Seven patients with QA failures/warnings from our calculation-based IMRT/VMAT QA were selected. Dose to 95% of PTV volume (D95%) and mean PTV dose with a difference of greater than 3%, a global gamma passing rate of less than 95%, and a mean dose difference to organ at risk (OAR) larger than 5% were set as the failure/warning criteria. The treatment sites of the selected patients included pelvis, leg, head and neck, brain, and breast. Measurement-based IMRT/VMAT QA was done on each patient using MapCHECK2. Root cause analysis (RCA) was performed on all patients to investigate the IMRT/VMAT failures/warnings.

Results: All seven patients’ plans passed measurement-based IMRT/VMAT QA with a gamma passing rate of more than 95%. Our RCA of patients showed the IMRT/VMAT QA failures detected by the calculation-based method were mainly due to inclusion of the skin in PTV, and tissue heterogeneity effects particularly at tissue-air and tissue-bone interfaces.

Conclusion: Calculation-based IMRT/VMAT QA can detect potential failures which cannot be detected by standard measurement-based QA.

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