SU-E-T-708: Spatial Accuracy of Small-Target SRS Using Conebeam CT Guidance and Static Arcs with MLC-Defined Fields
Purpose: To measure the spatial accuracy of dose delivery under conebeam CT guidance to small targets using static arcs with MLC-defined fields. Methods: A plastic (PMMA) phantom simulating a small brain lesion was constructed. The lesion was a cylindrical air cavity having 3 mm length and 3.175 mm diameter. Gafchromic EBT 2 film passing through the cavity was pin pricked at the exact geometric cavity center. Following CT simulation treatment plans involving static arcs with MLC-defined fields were designed, similar to radiotherapy with circular tertiary cones. 5×2 mm fields were generated by opening two centrally located leaf pairs of a high-definition MLC to 2.0 mm. The target was irradiated with 6 MV photons on a STx linac under conebeam CT image guidance to 600 cGy at isocenter. Films from coronal and sagittal planes were scanned and evaluated using the ImageJ software. The distance between the centroid of the 50% isodose line and the center of the pin prick was the measure of geographic accuracy. Results: Single arcs resulted in ellipsoidal 50% isodose surfaces having diameters of 5.0 mm along the gantry axis and 7.7 mm in the cross-sectional directions. 4-arc delivery at couch angles of 0, 45, 90 and 315 degrees produced 50% isodose surfaces having diameters of 5.6, 5.7 and 7.5 mm diameters along the lateral, longitudinal and anterior-posterior directions, respectively. In any of the experiments, the largest distance by which the centroid of the 50% isodose line missed the center of the target was 0.36 mm or less. Conclusion: MLC-defined static arcs delivered under conebeam CT guidance provide precise irradiation of small targets. The method is very efficient since no special attachments or modifications of the accelerator are required. Since MLC leaves are stationary throughout the treatment, patient-specific QA involving phantoms should not be required.