3307 - Anatomy-Guided Maximized-Volume Spatially Fractionated Radiotherapy Versus Conventional SABR in Oligometastatic Renal Cell Carcinoma: A Retrospective Review of a Prospective Cohort Study

Purpose/Objective(s): A substantial proportion of oligometastatic renal cell carcinoma (RCC) lesions are anatomically unsuitable for uniform ablative stereotactic ablative body radiotherapy (SABR) due to tumor size or proximity to critical organs-at-risk (OARs). Maximized-Volume Spatially Fractionated Radiotherapy (MV-SFRT), an anatomy-guided, partially ablative technique, may extend the feasibility of radical local therapy in this setting. This study aimed to compare the efficacy and safety of MV-SFRT versus standard SABR, each delivered concurrently with VEGF-targeted therapy and anti–PD-1 immunotherapy, in patients with oligometastatic RCC.

Materials/Methods: We conducted a retrospective review of a prospective cohort study of 68 patients with 1–5 metastatic RCC lesions treated between 2020 and 2025. All patients received concurrent targeted therapy and anti–PD-1 immunotherapy. MV-SFRT consisted of a stereotactic boost (6–8 Gy × 3–5 fractions) to the central tumor volume followed by conventionally or moderately fractionated radiotherapy (50-60 Gy total) to the full planning target volume. The primary outcome is progression-free survival (PFS), and other outcomes include overall survival (OS), local control (LC), objective response rate (ORR), disease control rate (DCR), and treatment-related toxicity. Inverse probability of treatment weighting (IPTW) was performed to adjust for baseline imbalances.

Results: Among 68 patients (median follow-up 25.2 months), 22 were treated with MV-SFRT and 46 with SABR. ORR (77.3% vs. 73.9%) and DCR (90.9% vs. 89.1%) were comparable between the two groups (P>0.05). The 2-year LC was 95.5% for MV-SFRT and 100% for SABR in the overall cohort (P=0.21). Following IPTW, median PFS was 30.8 months with MV-SFRT versus 29.4 months with SABR (HR 0.86, 95% CI 0.38–1.98; P=0.729), and median OS was not reached in either group (2-year OS 92% vs. 96.3%; HR 1.16, 95% CI 0.32–4.22; P=0.826), the 2-year LC rates were 93% and 100%, respectively (P<0.001). No grade =3 toxicities were seen secondary to radiotherapy.

Conclusion: MV-SFRT provided survival outcomes (PFS, OS) comparable to standard SABR, establishing it as a viable and isotoxic alternative for anatomically complex oligometastatic RCC lesions unsuitable for uniform ablative dosing. Although standard SABR has statistically significant higher LC, the rates are very high in both arms with small absolute differences. MV-SFRT successfully extended the feasibility of radical radiotherapy to otherwise ineligible patients within a modern combination therapy paradigm. Future prospective trials are warranted to validate these findings.