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Sensitivity of Quality Assurance Tools to Delivery Errors On a Magnetic Resonance-Imaging Guided Radiation Therapy (MR-IGRT) System

V Rodriguez

V Rodriguez1*, H Li1 , D Yang1 , R Kashani1 , H Wooten1 , J Dempsey2 , S Mutic1 , O Green1 , (1) Washington University School of Medicine, St. Louis, MO, (2) ViewRay Incorporated, Oakwood Village, OH,


SU-E-T-442 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To test the sensitivity of the quality assurance (QA) tools actively used on a clinical MR-IGRT system for potential delivery errors.

Methods: Patient-specific QA procedures have been implemented for a commercially available Cobalt-60 MR-IGRT system. The QA tools utilized were a MR-compatible cylindrical diode-array detector (ArcCHECK) with a custom insert which positions an ionization chamber (Exradin A18) in the middle of the device, as well as an in-house treatment delivery verification program. These tools were tested to investigate their sensitivity to delivery errors. For the ArcCHECK and ion chamber, a baseline was established with a static field irradiation to a known dose. Variations of the baseline were investigated which included rotated gantry, altered field size, directional shifts, and different delivery time. In addition, similar variations were tested with the automated delivery verification program that compared the treatment parameters in the machine delivery logs to the ones in the plan. To test the software, a 3-field conformal plan was generated as the baseline.

Results: ArcCHECK noted at least a 13% decrease in passing rate from baseline in the following scenarios: gantry rotation of 1 degree from plan, 5mm change in field size, 2mm lateral shift, and delivery time decrease. Ion chamber measurements remained consistent for these variations except for the 5 second decrease in delivery time scenario which resulted in an 8% difference from baseline. The delivery verification software was able to detect and report the simulated errors such as when the gantry was rotated by 0.6 degrees, the beam weighting was changed by a percent, a single multileaf collimator was moved by 1cm, and the dose was changed from 2 to 1.8Gy.

Conclusion: The results show that the current tools used for patient specific QA are capable of detecting small errors in RT delivery with presence of magnetic field.

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