Small cell carcinomas of the prostate gland are rare, and their histogenesis and clinical behavior remain poorly defined. We report a case with antidiuretic hormone secretion, which demonstrates direct transformation of the adenocarcinoma into the small cell component. The adenocarcinoma reacted positively for prostatic antigen, and negatively for carcinoembryonic antigen and neuron specific enolase, whereas the small cell component was negative for prostatic antigen, and positive for carcinoembryonic antigen and neuron specific enolase. At biopsy this was interpreted as denoting 2 separate tumors: one of prostatic and the other of nonprostatic origin. The clinical course was rapidly fatal but otherwise manifested the metastatic pattern of prostatic carcinoma. We caution that immunohistochemical reactions may be misleading if not interpreted in the context of other findings in the case. This case is labeled as a small cell carcinoma rather than a poorly differentiated adenocarcinoma of the ordinary type because the tumor exhibited morphological, immunohistochemical and biological features typical for that neoplasm.
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