Special AT-rich sequence-binding protein 2-positive primary small cell neuroendocrine carcinoma of the urinary bladder: A diagnostic pitfall.

Special AT-rich sequence-binding protein 2 (SATB2) expression in conjunction with CK20 expression has been established as a specific marker for the detection of colorectal adenocarcinomas. Recently, its expression in foregut, midgut, and hindgut neuroendocrine neoplasms has been documented. SATB2 expression has been documented in neuroendocrine neoplasms from various primary sites; however, its expression in the urinary bladder has not been reported. A case of primary small cell neuroendocrine carcinoma of the urinary bladder (SmCCB) is presented, where strong SATB2 expression led to the suspicion of metastatic neuroendocrine carcinomatous deposits from the lower gastrointestinal tract. However, on positron emission tomography-computed tomography scan, no fluorodeoxyglucose-avid lesion elsewhere in the body was found, which established the diagnosis of primary SmCCB with SATB2 expression.