Cytoreductive stereotactic body radiotherapy (SBRT) and combination SBRT with immune checkpoint inhibitors in metastatic renal cell carcinoma

Introduction: Preclinical evidence demonstrates the immunogenic potential of stereotactic body radiotherapy (SBRT). There is growing interest in investigating this interplay with the immune system in metastatic renal cell carcinoma (mRCC). Cytoreduction with SBRT and combination therapy with SBRT and checkpoint inhibitor immuno-oncology agents (IO) are two potential therapeutic strategies in mRCC. In this review, we summarize the current clinical evidence for the use of cytoreductive SBRT to primary kidney and combination SBRT with IO. Methods: A literature review for articles and abstracts published between January 2000 and March 2020 was conducted through the PubMed, the American Society of Clinical Oncology (ASCO), and the American Society of Radiation Oncology (ASTRO) databases. Evaluation of studies followed the Preferred Reporting Items for Systematic Reviews and Meta Analyses (PRISMA) criteria. Results: A total of three articles for cytoreductive SBRT and one article and three abstracts for combination SBRT and IO in mRCC met inclusion criteria for this review. Evidence for SBRT to primary kidney is limited by small series and pilot studies. Outcomes vary widely due to small patient numbers and study heterogeneity. Local control ranges from 85-100% and one-and two-year overall survival ranges from 38-71% and 19-53%, respectively. Combination SBRT and IO are tolerable for patients with early data, suggesting grade 3-4 adverse event rates of 0-24%. Long-term survival data is not yet available. Conclusions: Cytoreductive SBRT and combination SBRT with IO therapy represent promising treatment strategies in mRCC. The evidence for clinical benefit is currently limited and requires further study with well-designed, randomized controlled trials.