Dosimetric Evaluation of MapCHECK in MapPHAN for Conformal Arc SABR Quality Assurance

Abstract: This study investigates the sensitivity and specificity of the MapCHECK phantom housed in MapPHAN to errors in conformal arc Stereotactic Ablative Body Radiotherapy (SABR). A specific focus is applied to Organ At Risk (OAR) dosimetry. Multi-Leaf Collimator (MLC) class shift errors up to 2 mm, and isocentre shift errors of up to 1 mm, were introduced to 23 simulated 6 MV X-ray conformal arc lung SABR plans in Raystation. Overall, 198 plans were delivered on a Varian Clinac iX linear accelerator to an in-house modified MapCHECK in MapPHAN. Gamma analysis was used to compare the MapCHECK measurements to the simulated Raystation plans. Receiver Operating Characteristic (ROC) curves were generated from these results to determine the sensitivity and specificity of the measurement technique to the introduced errors. For MapCHECK in MapPHAN, a combination of 5%/1 mm with 95% gamma tolerance and 2%/1 mm with 90% tolerance provides good sensitivity and specificity for Quality Assurance (QA) of conformal arc SABR plans.