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Quality Assurance for IMRT/VMAT QA Devices: Issues Affecting the Timing for ArcCHECK Recalibration

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J Steers

J Steers1,2*, B Fraass1 , (1) Cedars Sinai Medical Center, Los Angeles, CA, (2) University of California-Los Angeles, Los Angeles, CA


SU-F-T-282 (Sunday, July 31, 2016) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose:To discuss several factors surrounding the decision on when to recalibrate the ArcCHECK device as well as present a simple and efficient monthly check to evaluate ArcCHECK calibrations.

Methods: ArcCheck (Sun Nuclear) calibrations were evaluated monthly by measuring a 25x25cm² field with 100 MU. Since ArcCHECK measurements are run on an almost nightly basis, such additional square field measurements are obtained with minimal additional effort. An in-house MATLAB script compares two radial (y-direction) profiles from the top/center of the new measurement relative to a baseline measurement acquired at the last device calibration. The program automatically generates PDF profile and percent difference comparisons for inspection. Recalibration is based on inspection of measurement profile shapes and percent differences from the baseline measurement.

Results: The method presented here shows the utility of a simple monthly check for evaluating ArcCHECK calibrations, and in addition shows the importance of recalibrating after Linac beam steering. Our device required recalibration approximately every 8-10 months. However, for ease of scheduling, we propose a bi-annual recalibration interval. Clinics with a lighter/heavier IMRT/VMAT QA case load may require different recalibration intervals, which are easily determined using the single-field method presented. Analysis of additional square fields is also easily incorporated, if desired. We further illustrate the importance of array recalibration given that diode irradiation is not uniform over the entire device, with central diodes receiving more than 900 Gy over the course of 10 months and peripheral diodes receiving as little as 50 Gy (in our experience). Finally, we show that timely device recalibration decreases spread in clinical IMRT/VMAT QA gamma passing rates.

Conclusion: Quality assurance for ArcCHECK array calibrations is important to ensure quality IMRT/VMAT QA comparisons. For many clinics, calibrations should be performed at least annually, if not more frequently, based on use of the proposed monthly check.

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