SBRT + Immuno-Oncology Fails to Boost RCC PFS

The evolving landscape of renal cell carcinoma (RCC) treatment saw a nuanced development presented at the 2026 ASCO Genitourinary Cancers Symposium: the addition of stereotactic body radiation therapy (SBRT) to first-line nivolumab and ipilimumab did not significantly improve progression-free survival (PFS) in the phase 2 CYTOSHRINK trial. While not a breakthrough, the study’s findings – and the safety profile observed – are crucial as clinicians grapple with optimizing immunotherapy regimens for this challenging cancer. This is particularly relevant given the increasing reliance on immunotherapy combinations as standard of care, and the ongoing search for strategies to deepen response and overcome resistance.

The CYTOSHRINK trial, conducted across Canada and Australia, randomized patients with de novo metastatic RCC to receive either nivolumab/ipilimumab alone or in combination with SBRT delivered to the primary kidney mass. The rationale behind adding SBRT was to potentially induce an abscopal effect – a phenomenon where local radiation triggers a systemic anti-tumor immune response. RCC has historically been a difficult cancer to treat, with limited options prior to the advent of immunotherapy. The combination of nivolumab and ipilimumab has become a cornerstone of first-line therapy, but a significant proportion of patients still do not experience durable responses, fueling the search for synergistic strategies.

The study’s results showed a median PFS of 10.2 months in the control arm versus 6.3 months in the SBRT arm, a difference that wasn’t statistically significant (HR, 1.20; P = .56). While the 12-month PFS rates also favored the control arm (47.8% vs 34.9%), this difference wasn’t conclusive. Interestingly, the objective response rate (ORR) was slightly higher in the SBRT arm (41.6% vs 32.5%), but ongoing responses were more frequently observed in the control group. Importantly, overall survival (OS) was not reached in either arm, and 1-year OS rates were high, suggesting that both regimens provide substantial benefit.

The Forward Look

Despite the lack of a statistically significant PFS improvement, the CYTOSHRINK trial doesn’t necessarily close the door on the potential of combining SBRT with immunotherapy in RCC. The observed baseline imbalances – particularly regarding T stage disease and the presence of liver metastases – are critical. Future studies will need to address these imbalances through more rigorous patient stratification or adaptive trial designs. Furthermore, the trial’s phase 2 nature means the findings are hypothesis-generating rather than definitive. Larger, phase 3 trials are needed to confirm these results and explore potential biomarkers that could predict which patients are most likely to benefit from SBRT in combination with nivolumab/ipilimumab.

We can anticipate a shift in focus towards identifying optimal SBRT parameters – including dose, fractionation, and target volume – and investigating the potential of combining SBRT with other immunotherapies or targeted agents. The safety profile observed in CYTOSHRINK is encouraging, suggesting that SBRT can be safely integrated into first-line treatment regimens. The next wave of research will likely concentrate on refining patient selection and optimizing treatment strategies to maximize the potential of this multimodal approach. The ongoing search for ways to enhance immunotherapy efficacy in RCC remains a high priority, and SBRT will undoubtedly continue to be a subject of intense investigation.

Reference

Lalani AK, Pond GR, Siva S, et al. CYTOSHRINK: a randomized phase II trial of cytoreductive stereotactic hypofractionated radiotherapy with ipilimumab/nivolumab for metastatic kidney cancer. J Clin Oncol. 2026;44(suppl 7):416. doi:10.1200/JCO.2026.44.7_suppl.416