Rare Early Coexistence of Classic Urothelial and Small Cell Carcinoma of the Urinary Bladder.

Introduction: Small cell carcinoma of the bladder (SCCB) is a rare aggressive cancer, representing less than 1% of bladder cancers. Similar to small cell lung cancer (SCLC), it progresses rapidly, metastasizes early, and has a poor prognosis, complicating treatment. Unlike more common bladder cancers, SCCB often resists localized treatments and responds better to chemotherapy. However, its management generally relies on protocols for SCLC due to limited specific research. Case Presentation: A 47-year-old male from Bangladesh presented with intermittent painless hematuria, weak urinary stream, and incomplete voiding for 2 months. His medical history includes diabetes mellitus, chronic kidney disease, hypothyroidism, adrenal adenoma, and coronary artery disease. His physical examination was unremarkable. His blood work up showed hypochromic microcytic anemia and elevated creatinine. Urine cytology revealed atypical degenerated urothelial cells, polymorphonuclear leukocytes, and red blood cells. He underwent flexible cystoscopy, which showed a nodular tumor with a wide base at the left lateral bladder wall measuring less than 3 cm. He had a CT scan, which revealed irregular thickening in the left posterolateral bladder wall, suggesting muscle-invasive carcinoma extending into the perivesical fat. He had an MRI scan which confirmed a 4 x 4.5 - cm mass with extravesical involvement. Histopathology examination from his transurethral resection of bladder tumor specimens showed mixed carcinoma, comprising both classic invasive urothelial carcinoma (UC) and SCCB. The small cell component exhibited scant cytoplasm, high nuclear-to-cytoplasmic ratio, hyperchromatic nuclei, and numerous mitotic figures, with synaptophysin and weak chromogranin positivity; UC cells were GATA-3 positive. Surgical resection removed most of the tumor. The patient tolerated the procedure well, with no complications. After four chemotherapy cycles, he had a follow-up PET CT, which showed significant tumor regression. Conclusion: This case highlights the unusual early coexistence of SCCB with classic UC in the bladder, emphasizing the importance of comprehensive histopathological and immunohistochemical evaluation for accurate diagnosis and treatment planning. Further research on SCCB could improve diagnosis, treatment strategies, and identification of molecular targets, potentially enhancing survival rates and patient quality of life.