[A case of small cell carcinoma in a diverticulum of the bladder].

We report a case of small cell carcinoma in a diverticulum of the bladder. A 64-year-old Japanese man visited our hospital with the chief complaint of urinary retention. Cystoscopy revealed a papillary tumor arising in a diverticulum on the right lateral wall of the bladder. Partial cystectomy was performed under the diagnosis of bladder cancer in the diverticulum. The histopathological diagnosis was a combination of small cell carcinoma and urothelial carcinoma, of which the former was dominant. It is often difficult to detect tumors in the bladder diverticula and its early penetration is not rare because of the thin wall. Therefore, the prognosis of the bladder cancer in the diverticulum is believed to be generally poor. On the other hand, small cell carcinoma originating in the urinary bladder is also characterized by its extreme rareness and poor prognosis. To our knowledge, this is the second case report of the small cell carcinoma arising in a diverticulum of the urinary bladder in the Japanese literature.