Case ReviewsDiffuse Synaptophysin Expression in Aggressive Recurrent Castration-Resistant Acinar (Usual) Prostate Cancer Case Report and Review of Prostatic Neuroendocrine TumorsRussell, Daniel H. MD; Mignano, Salvatore E. DOAuthor Information From the Department of Pathology: Anatomic Pathology, Tripler Army Medical Center, Honolulu, HI. Reprints: Daniel H. Russell, MD, Department of Pathology: Anatomic Pathology, Tripler Army Medical Center, Room 2A109, 1 Jarrett White Rd, Honolulu, HI 96859. E-mail: firstname.lastname@example.org. The authors have no funding or conflicts to declare. The views expressed in this abstract/manuscript are those of the author(s) and do not reflect the official policy or position of the Department of the Army, Department of Defense, or the US Government. AJSP: Reviews & Reports: May/June 2020 - Volume 25 - Issue 3 - p 114-122 doi: 10.1097/PCR.0000000000000354 Buy Metrics 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. © 2020 Lippincott Williams & Wilkins, Inc.