Diffuse Synaptophysin Expression in Aggressive Recurrent Castration-Resistant Acinar (Usual) Prostate Cancer: Case Report and Review of Prostatic Neuroendocrine Tumors

Abstract Metastatic or locally advanced prostate cancer in a majority of men recurs within two to three years. The literature contains ample evidence that locally advanced and metastatic prostate cancers are associated with increased neuroendocrine expressing tumor cells, as is castration resistance. Usual (acinar) prostatic adenocarcinoma with neuroendocrine features is a high-grade malignant acinar tumor with neuroendocrine expression, which can be mistaken for neuroendocrine carcinoma, in particular large cell neuroendocrine carcinoma. We present a case of metastatic acinar prostatic adenocarcinoma with neuroendocrine features with strong and diffuse synaptophysin expression, mimicking a metastatic neuroendocrine tumor. We review recognized prostatic neuroendocrine tumors, tumors with neuroendocrine differentiation requiring further investigation, relevant differential considerations, and recommended ancillary workup of challenging cases.