An Unexpected and Unpredictable Emergence of Muscle-Invasive Bladder Cancer in a Patient Successfully Treated With Nivolumab for Small-Cell Bladder Cancer.

Small-cell bladder cancer (SCBC) is a rare subtype of bladder cancer with aggressive behavior and poor prognosis. Here, we report the case of a 50-year-old man who presented with hematuria for one month. A computed tomography scan showed an exophytic lesion on the right posterolateral wall of the bladder and a single liver metastasis with a 14 mm diameter. Transurethral resection of the bladder tumor was performed, and postoperative examination of the specimen showed muscle-invasive SCBC. Initially, the patient was treated with neoadjuvant chemotherapy. Rapid clinical and imaging deterioration was observed after the premature end of cisplatin and etoposide therapy. Second-line therapy with nivolumab demonstrated systemic and local complete response. However, the patient was further diagnosed with unpredictable and unexpected urothelial muscle-invasive bladder cancer. After 76 months of regular follow-up, imaging workup did not demonstrate SCBC recurrence or urothelial bladder cancer progression. This report highlights this disease's rarity and severity and no typical or even pathognomonic clinical and radiological presentation. Therefore, histopathology and immunohistochemistry findings play a key role in diagnosis. Immunotherapy has opened a new window in cancer treatment and maybe SCBC patients can benefit from it.