Session Item

To establish the action levels for pre-treatment QA of single-isocenter multiple brain metastasis stereotactic radiosurgery (SRS) plans using a new 2D detector array.

The Octavius 4D Modular Phantom (O4D) with Top SRS plus phantom. Fifteen clinically treated SIMT SRS plans were retrospectively included in this study. Number of lesions per plan ranged from 2 to 6 (median: 2), with a median volume of 1.5 cm³. Distance from target center to treatment isocenter ranged from 1.7 to 6.2 cm (median: 6.2 cm). Plans were designed with the Eclipse TPS (AAA v 10.0.28, 1 mm calculation grid size) using non-coplanar IMRT technique, and delivered using 6 MV photons from a Varian Clinac 2100 CD linac with a Millennium 120 MLC.

Each clinical SRS plan was recalculated on the Top SRS plus phantom synthetically created in the Eclipse. From the measurements taken by the 2D array for the different beam incidences of the SRS plan, the PTW Verisoft software (v 8.0.0164) reconstructs the 3D dose within a cylindrical region of 15 cm-diameter contained by the Top SRS plus phantom. Dose reconstruction with 1 mm voxel size was selected in Verisoft. Eclipse and Octavius-based dose distributions were compared using the 3D gamma analysis tool of the Verisoft software. The global (G) criteria used for the gamma analysis were 3%(G)/1 mm and 3%(G)/2 mm, for 10% and 70% dose thresholds. The 70% threshold focused the analysis in the high dose regions that approximately correspond to the targets, while the 10% one is the typical dose cut off used for the gamma analysis. 3D gamma passing rates (GPRs) and confidence levels (CLs) for the GPR metric were derived.

Gamma passing rates (GPRs) are shown in Table 1.  CLs for 3%(G)/1 mm criteria were 90.5% and 57.8% for the 10% and 70% dose thresholds, respectively.  CLs for 3%(G)/2 mm  criteria were 95.9% and 95.3% for the 10% and 70% dose thresholds, respectively.

Acceptable CLs were obtained for 3%(G)/2 mm criteria. 1 mm distance-to-agreement seems too tight to be used as tolerance with the Octavius Detector 1600 SRS. Based on our evaluation, we have set our departmental criteria at 3%(G)/2 mm for a 90% pass regardless the dose threshold (10% and 70%), when the Octavius Detector 1600 SRS is used.