Purpose: Stereotactic radiosurgery (SRS) for multiple targets on LINACs often requires complex MLC motions that generate highly conformal dose distributions with intended submillimeter precision. High resolution planar or volumetric detectors are essential tools for commissioning, and on-going validation of LINAC-based SRS plans. We evaluated performance of a 2D-CMOS detector-array (MyQA SRS, IBA Dosimetry, Germany) and its dependencies for patient-specific quality assurance of SRS plans.
Methods: Detector correction factors including response-equalization, linearity, repetition rate, field size and angular dependence were measured for 6xFFF beam. A look-up-table (LUT) for various couch and gantry angles was constructed using corresponding ion chamber measurements to correct array response. End-to-end tests for seven patients compared dose distributions obtained by EBT-XD film with the array using 1mm distance-to-agreement and dose-differences of 3% gamma criteria.
Results: Operating temperature of the system stabilized after 30 minutes from 25 to 32 °C. During this period, a 1.5% fluctuation was observed in the detector response. At couch-0, gantry-angular correction factors increased from 1 (Gantry-0) to 1.36 (Gantry-90) suggesting significant angular-response dependence. Similarly, maximum correction factors at couch-30 and 60 were 1.38 and 1.4. While variations as large as 6% in correction factors were observed, the gantry-angle range for deviations beyond 3% was found to span within 15◦ from the horizontal direction (90°). As such, the impact of these variations on the measured dose difference for SRS arcs which typically span >120° was negligible. For the seven patients considered in this study, field-by-field gamma pass rates with respect to film ranged between 82-100% with an average gamma pass rate of 94%.
Conclusion: Utilization of an angular correction LUT enables accurate measurement of field doses at a range of gantry-couch positions. Additionally, the submillimeter resolution of the CMOS detector array enables meaningful quality assurance of dose distributions that span only a few millimeters.
Stereotactic Radiosurgery, Dosimetry, Resolution
TH- External Beam- Photons: Small field experimental dosimetry