Author: Asma Amjad, Slade J. Klawikowski, Natalya V. Morrow, Haidy G. Nasief, Eric S. Paulson, An Tai, Hualiang Zhong 👨🔬
Affiliation: Department of Radiation Oncology, Medical College of Wisconsin 🌍
Purpose: Accurate and precise linac-based SRS commissioning can be very challenging. Thus, it is important to increase the confidence in the measurement at each step prior to end-to-end testing. The purpose of this study is to document the steps used for streamlined SRS commissioning of an Elekta Linac to increase efficiency, accuracy, and precision
Methods: SRS commissioning was performed on 6MVFFF beam with Agility MLC on Elekta VersaHD linac equipped with Hexapod couch. Relative output factor (ROF), percentage depth dose and profile measurements were performed in a 48×48×40cm3 IBA_Blue water tank using: Exradin W2-scintillation, A16- and Edge-detectors. Measurements were validated against Monte Carlo beam data in the Monaco treatment planning system(TPS) using commercially available Monaco-utility tool with 2%/1mm and 3%/1mm gamma analysis and cross-validated among three detectors. Film dosimetry using Gafchromic film was established to verify ROF measurements. Patient-specific QA with and without couch-kicks was performed and evaluated with Sun Nuclear SRS-Mapcheck using 3%/1mm criteria. Agreement between imaging- and radiation-isocenters was evaluated using Sun Nuclear Multi-Met Winston-Lutz (WL) cube with the kV-imaging isocenter calibrated to match 6FFF radiation isocenter. The IROC_SRS Anthropomorphic Phantom was irradiated for end-end testing. Treatment uncertainties from all sources were estimated. Real-time patient monitoring during SRS using VisionRT was evaluated.
Results: Monaco TPS calculations showed >95% gamma passing rates for field sizes as small as 0.6 × 0.6 cm² compared to measured data. ROF for all field sizes were within 2% of the TPS model. The Exradin W2 scintillator demonstrated superior performance for small-field dosimetry. Six patient-specific measurements achieved > 95% passing rates. Motion is successfully tracked with VisionRT. Correct mask making is crucial to limit the motion to <1mm. The WL-test achieved field-target coincidence within 1mm, and IROC SRS phantom test successfully passed.
Conclusion: A rigorous commissioning program enables safe and precise SRS treatments with an Elekta Versa Linac.