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Sensitivity of ArcCheck to Delivery Errors in IMRT/VMAT Treatment

W Yang

W Yang*, R Wallace, K Huang, R Cook, B Fraass, Cedars-Sinai Medical Center, LOS ANGELES, CA

SU-E-T-363 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

Purpose: There is an increasing trend to perform IMRT QA measurements using ion chamber or diode arrays in place of film. This change has improved QA efficiency but raises questions about the system's sensitivity to machine mechanical accuracies due to their coarse resolution. This work uses intentional set up and mechanical errors to test the sensitivity of a cylindrical IMRT QA detector array to operator and machine errors.

Methods: A cylindrical phantom (Sun Nuclear ArcCheck) with a 3D array of 1385 diodes, arranged in a spiral pattern with 10 mm detector spacing, was studied. Translational, rotational (via collimator rotation) and multi-leaf collimator positioning errors were deliberately induced and applied. Translational shifts of 2-10 mm were applied on a VMAT plan. Collimator rotations up to 5o were applied to a 10 cm open anterior-posterior field. 1-3mm systemic shifts were applied to all MLC leaves in the same bank in an IMRT plan. Gamma analysis of varying thresholds was used to analyze the results.

Results: ArcCheck is sensitive to <1 mm translational shifts in all major axes, resulting in a 20% to 60% drop in passing rates for 2% tolerance, with much higher sensitivity to longitudinal shifts. Collimator rotation of 1o is detectable in the static 10 cm open field test. The systemic MLC positioning errors could be detected in gamma tests with tolerances ranging from 1%/1mm to 3%/3mm.

Conclusions: Despite the 1 cm detector spacing, ArcCheck is sensitive to small set-up and systematic MLC shifts on the order of 1 mm. It is also sensitive to small (1 degree) rotational errors in the setup field. The spiral placement geometry and large number of the detectors statistically helps compensate for the relatively coarse resolution of the detectors. The study shows that it is possible to tighten the criteria in IMRT/VMAT patient QA.

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