SU‐GG‐T‐247: A New Method to Compensate Angular Dependency of MapCheck Device in Intensity Modulated Arc Therapy

Purpose: To evaluate the performance of a new method to compensate angular dependency of QA devices in intensity modulated arc therapy. Methods and Materials: The QA phantom consisted of a MapCheck device sitting in an acrylic base, which is inserted between two 5‐cm solid water pieces. Two‐dimensional dose planes were measured by delivering one 10×10cm 6MV X‐ray beam for per 15 degree beam angle. Two‐dimensional doses at the diode detector plane were calculated in Eclipse using the AAA algorithm with inhomogeneity correction. Two types of halfpipe‐shape boluses were added to the planning CT image. Bolus A has uniform thickness to attenuate the TPS dose to match the measured dose at the 180 degree beam angle. Bolus B has variable thickness to match the doses at every 15 degree beam angle. TPS doses were re‐calculated using the modified CT images. Angular dependences were evaluated for the original phantom and with bolus A and B individually. Six patient QAs were performed to compare gamma analysis results. Results: Without compensation, the TPS dose was about 5% higher than the measured dose for most posterior angles (105 degree to 180 degree). It is probably due to angular dependence of diode detectors and CT metal artifacts. Bolus A slightly reduced the angular dependence for the posterior angles (RMS decreased from 4.0% to 3.4%). Bolus B reduced the dependence significantly (RMS=1.4%). The failure rate of gamma analysis was reduced by 39% and 41% in average. Conclusion: The new method has been shown to be able to compensate angular dependency for the MapCheck device.