Durable Response to Immune Checkpoint Inhibition in Small Cell Carcinoma of the Bladder: A Case-Based Review of Immunotherapy and CNS Relapse

Background

Small cell carcinoma of the bladder (SCCB) is a rare, aggressive malignancy with no established standard of care for metastatic disease. The prognosis is dismal; median survival ranges from 12 to 20 months, and long-term survival is exceptional. Case Presentation: We present a 67-year-old man diagnosed in 2018 with metastatic SCCB involving the liver, bone, and ureter. He achieved a complete metabolic response following six cycles of cisplatin–etoposide, followed by consolidative pelvic radiotherapy with a bladder boost. Upon disease progression in 2019, molecular profiling identified a high tumour mutational burden (19.9 mut/Mb), PD-L1 CPS > 1%, and multiple genomic alterations (including MDM2, RAD51D, and TERT). Dual immune checkpoint inhibition (ICI) with ipilimumab and nivolumab was initiated. Although treatment was interrupted and subsequently discontinued due to grade 3 immune-related adverse events, including hepatitis and pneumonitis, the patient maintained a sustained remission off therapy. In January 2025, nearly seven years post-diagnosis, he developed a solitary frontal brain metastasis. He underwent resection and adjuvant stereotactic radiosurgery, achieving local control. He remains under surveillance with no evidence of extracranial recurrence.