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

<b>Background/Objectives:</b> 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). <b>Methods:</b> 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. <b>Results:</b> 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. <b>Conclusions:</b> 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.