The goal of this multicenter observational study is to elucidate the clinical and immunological characteristics of the abscopal effect in patients with metastatic renal cell carcinoma (mRCC) receiving immune checkpoint inhibitors (ICI) combined with image-guided ultra-hypofractionated radiotherapy (IGU). The main questions this study aims to answer are: What is the abscopal response rate (ARR) at one year after IGU in patients continuing ICI treatment? What clinical and immunological factors are associated with the occurrence and timing of the abscopal effect? Participants are patients with mRCC who have experienced immune-confirmed stable or progressive disease during ICI therapy and are scheduled to receive IGU to a selected lesion. Researchers will observe tumor responses at irradiated and non-irradiated sites using standard imaging (CT/MRI) and collect clinical and laboratory data at baseline, 3, 6, 9, and 12 months after IGU. Optional exploratory blood samples will be obtained for cytokine analysis (e.g., IFN-β, IFN-γ, TNF-α, IL-6). The primary outcome is the abscopal response rate (ARR) at one year after IGU. Secondary outcomes include tumor shrinkage rate of irradiated and non-irradiated lesions, 1-year overall survival, disease-specific survival, and progression-free survival. This study seeks to establish a foundation for developing combined immunotherapy and ultra-hypofractionated radiotherapy strategies for metastatic renal cell carcinoma. \*This study is led by Prof. Hiroshi Onishi (University of Yamanashi). The registry entry is managed by Dr. Zhe Chen on behalf of the study group.