A Case of De Novo Neuroendocrine Prostate Cancer Presenting With Elevated Carcinoembryonic Antigen (CEA) Levels.

Neuroendocrine prostate cancer (NEPC) is an extremely rare and highly aggressive malignancy with a poor prognosis. We present the case of a 74-year-old man who presented with weight loss and a markedly elevated carcinoembryonic antigen (CEA) level. Fluorodeoxyglucose positron emission tomography/computed tomography (FDG-PET/CT) revealed a primary prostatic mass with widespread metastases. Despite a low prostate-specific antigen (PSA) level, neuron-specific enolase (NSE) and pro-gastrin-releasing peptide (ProGRP) levels were significantly elevated. Prostate biopsy with immunohistochemical analysis confirmed neuroendocrine carcinoma with positivity for CD56 and CEA, focal positivity for synaptophysin (Syn) and chromogranin A (CgA), and negativity for PSA. Due to poor performance status, chemotherapy could not be initiated, and the patient died one month following the biopsy. De novo NEPC should be considered in patients presenting with widespread metastatic disease and markedly elevated CEA levels in the context of low PSA. Early biopsy and immunohistochemical evaluation are essential for prompt diagnosis.