Significant Response to Nivolumab Plus Ipilimumab in Metastatic Mucinous Tubular and Spindle Cell Carcinoma: A Case Report

Introduction Mucinous tubular and spindle cell carcinoma (MTSCC) is a rare renal cell carcinoma subtype that is typically indolent but may exhibit aggressive behavior. No standard systemic therapy has been established for metastatic MTSCC. Case Presentation A 71‐year‐old woman underwent retroperitoneoscopic radical left nephrectomy for MTSCC (pT1b). Six months later, computed tomography (CT) and 18 F‐fluorodeoxyglucose positron emission tomography (PET)/CT revealed lung, bone, and adrenal metastases. Bone biopsy confirmed metastatic MTSCC, and the patient was classified as poor risk. Combination therapy with nivolumab and ipilimumab was initiated. After two cycles, PET/CT showed marked regression of metastatic lesions (SUV max decreased from 21.7 to 4.3). RECIST evaluation demonstrated a partial response. Genomic profiling revealed microsatellite stability and a low tumor mutational burden, with no clinically actionable alterations identified on FoundationOne CDx. No disease progression was observed at 9 months. Conclusion This case demonstrates the potential efficacy of combination immunotherapy in aggressive metastatic MTSCC.