Purpose: The current standard for patient-specific QA is based on AAPM Task Group Reports 120 and 218, which provide guidance on appropriate measurement devices, methods, and data comparison criteria. As the field bifurcates across EPID-based, array-based, and logfile-based methods, collaboration of experiences comparing them is necessary for refinement of current guidelines. The focus of this study was to compare EPID-based and array-based measurements to determine their threshold for MLC error detection.
Methods: Five VMAT fields with Millennium 120 MLCs were measured with Sun Nuclear MapCHECK3 (MC3) and Varian Portal Dosimetry (PD), using perpendicular field-by-field delivery. Initial plans were modified to include a single MLC error of increasing magnitude (0mm, 1mm, 2mm, 5mm, 10mm, 20mm) for all control points. The measured dose of each modified field was compared to the predicted dose of the original fields. Gamma comparison criteria of 3% dose difference and 2mm distance-to-agreement was considered passing when >95% of points contained a gamma value ≤1 (10% threshold, local comparison).
Results: A single MLC error of 0mm, 1mm, 2mm, 5mm, 10mm, and 20mm corresponded with average pass rates of 99.6%, 99.6%, 99.6%, 99.0%, 96.1%, and 83.5% using PD and 95.8%, 95.8%, 94.3%, 91.4%, 89.1%, and 78.3% using MC3. The MC3 calculated shift was capable of passing all fields. Observable differences in the measured and predicted isodose maps were present when an MLC error ≥2mm was introduced. PD and MC3 MLC error thresholds were determined to be ≥5mm and ≥2mm, respectively.
Conclusion: EPID and array-based methods of patient QA are widely used with gamma of 3% and 2mm, but this may not be optimal for MLC error detection. This investigation is ongoing to determine improved methods of MLC error detection and the dosimetric impact of such errors if undetected. Varying methods of gamma comparison among vendors will be further evaluated.
MLC, Quality Assurance, Radiation Detectors