Purpose: To improve linac-based SRS program, treatment delivery, and patient QA with goal of expanding SRS service in the system and create a standard for linac-based SRS planning utilizing the optimized model.
Methods: Compare small field (≤3x3cm²) profiles measured on Varian TrueBeam with an IBA Blue Phantom² water phantom using SNC Edge detector (measured) to those calculated in Eclipse TPS with an optimized Acuros 15.6 algorithm for small fields (optimized). In-water depth dose profiles were collected at 100cm SSD for 6X-FFF energy. Crossline profiles were collected at 100cm SSD at depths 1.5, 10, and 20cm for 6X-FFF. Small fields are symmetric and include 1x1, 2x2, and 3x3cm² delimited by multi-leaf collimator (Varian Millennium MLC 120) with X and Y collimator jaws static at 3x3cm². Optimization is achieved through iterative changes of dosimetric leaf gap (DLG) and assigning Effective Target Spot Size (ETSS) to X=1.5mm and Y=0.0mm as defined by Varian white paper “Using Varian Photon Beam Source Model for Dose Calculation of Small Fields.” Primary analysis includes comparison of field widths and penumbra of measured and optimized profiles. Test plans with dynamic beams will be tested in an end-to-end method using portal dosimetry and detector array.
Results: Optimization still in progress. An analysis of field widths and penumbra was performed for measured and optimized profiles and compared. Results from first algorithm iteration indicate a different DLG value is necessary to improve agreement between measured and optimized data.
Conclusion: Pilot data still being collected. Preliminary results indicate a good agreement between measured data and optimized profiles. This study will provide DLG and ETSS values that can be used by other centers attempting to implement linac-based SRS using Varian TrueBeam with Millennium MLC 120.