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

Abstract

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.

Keywords: MR-Linac; MRgRT; RCC; SBRT; UTUC; kidney.

Figure 1

Illustration of an online adaptive plan for MR-guided SBRT. (a) Reference plan shown on the simulation CT image. (b) The adaptive plan for the 3rd fraction shown on the daily MR image. (c) The dose-volume histogram (DVH) comparing the reference plan (dashed lines) and the adaptive plan (solid lines).

Figure 2

Examples of kidney target motion management with a compression belt (CB) in two patients undergoing magnetic resonance-guided stereotactic body radiation therapy. (a) Tumor motion without use of the belt was 18 mm for Patient 1 (top) and 13 mm for Patient 2 (bottom). (b) Tumor motion was significantly reduced with use of the belt to 7 mm for Patient 1 (with a left kidney tumor) and to 3 mm for Patient 2 (with a right kidney tumor), as shown by the dashed red lines and arrows.

Figure 3

Comparison of respiratory-induced target motion in 16 patients, with and without use of a compression belt (CB) during MR-guided SBRT. Blue indicates cranio–caudal motion without the belt; green shows motion with the belt; and yellow indicates the average daily motion with the belt applied during treatment sessions. The red dashed line represents the clinical threshold of 10 mm, below which motion is considered sufficiently controlled to allow MR-Linac treatment. Although some patients experienced greater daily motion, it generally fell below the clinical thresholds for treatment, particularly if a belt was used during simulation.