Upper Urinary Tract Stereotactic Body Radiotherapy Using a 1.5 Tesla Magnetic Resonance Imaging-Guided Linear Accelerator: Workflow and Physics Considerations.

Simple Summary: The MR-Linac (or MRL) is a powerful new device that integrates high-resolution magnetic resonance imaging (MRI) within a linear accelerator to enhance the precision of radiation treatment delivery beyond the predominantly CT-guided standard of care. Our institution was one of the seven founding members of the consortium that tested and refined the 1.5 Tesla MR-Linac in preparation for the first-in-human clinical trials, resulting in several years of early clinical experience. Its application in delivering ablative doses (stereotactic ablative radiation therapy; SBRT) to renal cell carcinoma (RCC) or upper tract urothelial carcinomas (UTUC) has been of particular interest out of clinical necessity and technical challenge. We present a retrospective analysis of our multi-year experience using MRL-SBRT, with emphasis on our evolving treatment setup and early clinical outcomes. Our aim is to contribute to and support the development and innovation of further programs using one of the largest worldwide single-institution cohorts. Background/Objectives: Advancements in radiotherapy technology now enable the delivery of ablative doses to targets in the upper urinary tract, including primary renal cell carcinoma (RCC) or upper tract urothelial carcinomas (UTUC), and secondary involvement by other histologies. Magnetic resonance imaging-guided linear accelerators (MR-Linacs) have shown promise to further improve the precision and adaptability of stereotactic body radiotherapy (SBRT). Methods: This single-institution retrospective study analyzed 34 patients (31 with upper urinary tract non-metastatic primaries [RCC or UTUC] and 3 with metastases of non-genitourinary histology) who received SBRT from August 2020 through September 2024 using a 1.5 Tesla MR-Linac system. Treatment plans were adjusted by using [online settings] for "adapt-to-position" (ATP) and "adapt-to-shape" (ATS) strategies for anatomic changes that developed during treatment; compression belts were used for motion management. Results: The median duration of treatment was 56 min overall and was significantly shorter using the adapt-to-position (ATP) (median 54 min, range 38–97 min) in comparison with adapt-to-shape (ATS) option (median 80, range 53–235 min). Most patients (77%) experienced self-resolving grade 1–2 acute radiation-induced toxicity; none had grade ≥ 3. Three participants (9%) experienced late grade 1–2 toxicity, potentially attributable to SBRT, with one (3%) experiencing grade 3. Conclusions: We conclude that MR-Linac-based SBRT, supported by online plan adaptation, is a feasible, safe, and highly precise treatment modality for the definitive management of select upper urinary tract lesions. [ABSTRACT FROM AUTHOR]