Adaptive-driven CT simulation-free multi-fraction SBRT radiotherapy: Initial clinical experience

Introduction: Using diagnostic CT for radiotherapy (RT) planning can bypass traditional CT simulation but introduces challenges in patient positioning and Hounsfield unit (HU) fidelity, affecting dose accuracy. Here a Varian Ethos adaptive-driven CT direct-to-treatment (DtT) multi-fraction stereotactic body radiation therapy (SBRT) workflow is presented. Methods: This study employed institutional diagnostic PET-CT images to generate an initial reference Ethos treatment plan. HU and dosimetric accuracy were validated for PET-CT, Ethos CBCT images (with and without Hypersight (HS), and the gold-standard helical CT simulators). Following the SBRT reference plan creation on the low dose diagnostic PET-CT, the first fraction was delivered with a newly generated adaptive plan calculated on the HS CBCT (Ethos) images. For multi-fraction treatments, the first day CBCT images and adaptive plan become the reference for subsequent IGRT treatments. This study includes workflow validation and initial three patient experience. Results: The DtT adaptive SBRT workflow was successfully implemented, with initial end-to-end testing demonstrating feasibility. In-house solutions were introduced to facilitate the adaptive to IGRT plan conversion. The Ethos system, especially with HS, maintained HU fidelity and dose calculation accuracy comparable to helical CTs. On-table adaptive sessions were within 37–51 min, aligning with single-fraction palliative studies. Subsequent non-adaptive IGRT fractions were efficiently completed within 7–27 min. Conclusions: This study demonstrates the feasibility of DtT adaptive-driven multifraction SBRT using diagnostic PET-CT. This approach reduces the need for on-site patient presence prior to treatment, expands planning lead times, and improves overall efficiency in radiotherapy for well selected patients.