{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,4,1]],"date-time":"2026-04-01T14:30:27Z","timestamp":1775053827316,"version":"3.50.1"},"reference-count":15,"publisher":"Frontiers Media SA","license":[{"start":{"date-parts":[[2026,4,1]],"date-time":"2026-04-01T00:00:00Z","timestamp":1775001600000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":["frontiersin.org"],"crossmark-restriction":true},"short-container-title":["Front. Oncol."],"abstract":"<jats:p>This case report describes an exceptionally rare manifestation of prostate adenocarcinoma presenting with peritoneal carcinomatosis, rectal obstruction, and markedly elevated carcinoembryonic antigen (CEA), closely mimicking colorectal cancer. A 65-year-old man with localized high-grade prostate adenocarcinoma (Gleason 4\u00a0+\u00a04) underwent robotic prostatectomy in November 2020, followed by salvage radiation and stereotactic body radiation therapy for early biochemical recurrence. He achieved an excellent response to androgen deprivation therapy (ADT) combined with abiraterone and prednisone, maintaining an undetectable prostate-specific antigen (PSA) level for over 2 years. In late 2024, the patient developed hematochezia, narrow stools, and abdominal\u00a0pain. Imaging revealed a rectal mass, diffuse peritoneal carcinomatosis, and new hepatic lesions. Despite an undetectable PSA, his serum CEA exceeded 7,000 ng\/mL, strongly suggesting a colorectal primary. Multiple biopsies and immunohistochemical stains were inconclusive, showing poorly differentiated adenocarcinoma without definitive prostate or colorectal markers. Given the clinical presentation, FOLFOX chemotherapy was initiated empirically for presumed colorectal carcinoma. Next-generation sequencing (NGS) of rectal tissue ultimately identified a TMPRSS2::ERG fusion identical to that in the original prostatectomy specimen, strongly supporting a diagnosis of metastatic prostate adenocarcinoma origin. A subsequent liver biopsy corroborated the findings. The patient transitioned to docetaxel plus ADT, resulting in symptomatic improvement and partial radiologic response. Despite temporary stabilization, progressive bowel dysfunction required a palliative colostomy. Omental biopsies again confirmed metastatic prostate adenocarcinoma. This report underscores several key clinical lessons. First, peritoneal metastasis from prostate cancer is exceedingly uncommon and can masquerade as gastrointestinal malignancy, particularly when accompanied by high CEA levels and rectal involvement. Second, standard markers such as PSA and immunohistochemistry may be misleading in atypical presentations. Finally, the case highlights the decisive role of molecular diagnostics, specifically NGS, in identifying tumor origin and preventing misdiagnosis.<\/jats:p>","DOI":"10.3389\/fonc.2026.1736081","type":"journal-article","created":{"date-parts":[[2026,4,1]],"date-time":"2026-04-01T12:26:37Z","timestamp":1775046397000},"update-policy":"https:\/\/doi.org\/10.3389\/crossmark-policy","source":"Crossref","is-referenced-by-count":0,"title":["Case Report: Prostate adenocarcinoma with peritoneal carcinomatosis and elevated CEA mimicking colorectal cancer: a diagnostic dilemma"],"prefix":"10.3389","volume":"16","author":[{"given":"Hala","family":"Majeed","sequence":"first","affiliation":[{"name":"Department of Medicine, Division of Medical Oncology, Markey Cancer Center, University of Kentucky","place":["Lexington, KY, United States"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Nathan","family":"Shelman","sequence":"additional","affiliation":[{"name":"Department of Pathology and Laboratory Medicine, University of Kentucky","place":["Lexington, KY, United States"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Jitesh A.","family":"Patel","sequence":"additional","affiliation":[{"name":"Department of Surgery, Division of Colorectal Surgery, Markey Cancer Center, University of Kentucky","place":["Lexington, KY, United States"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Janeesh S.","family":"Veedu","sequence":"additional","affiliation":[{"name":"Department of Medicine, Division of Medical Oncology, Markey Cancer Center, University of Kentucky","place":["Lexington, KY, United States"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Insija I.","family":"Selene","sequence":"additional","affiliation":[{"name":"Department of Medicine, Division of Medical Oncology, Markey Cancer Center, University of Kentucky","place":["Lexington, KY, United States"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Ashlyn","family":"Whitesell","sequence":"additional","affiliation":[{"name":"College of Pharmacy, Markey Cancer Center, University of Kentucky","place":["Lexington, KY, United States"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Zin W.","family":"Myint","sequence":"additional","affiliation":[{"name":"Department of Medicine, Division of Medical Oncology, Markey Cancer Center, University of Kentucky","place":["Lexington, KY, United States"]}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"1965","published-online":{"date-parts":[[2026,4,1]]},"reference":[{"key":"B1","volume-title":"Prostate cancer statistics","year":"2023"},{"key":"B2","doi-asserted-by":"publisher","DOI":"10.1002\/1097-0142(19841215)54:12<3078::aid-cncr2820541245>3.0.co;2-u","article-title":"Metastatic patterns of prostatic cancer: correlation between sites and number of organs involved","volume":"54","author":"Saitoh","year":"1984","journal-title":"Cancer"},{"key":"B3","doi-asserted-by":"publisher","DOI":"10.7759\/cureus.15569","article-title":"Rectal invasion by metastatic prostate adenocarcinoma","volume":"13","author":"Wadehra","year":"2021","journal-title":"Cureus"},{"key":"B4","doi-asserted-by":"publisher","DOI":"10.1093\/jscr\/rjae046","article-title":"Prostate carcinoma mimicking rectal cancer: a case report","volume":"2024","author":"Hassine","year":"2024","journal-title":"J Surg Case Rep"},{"key":"B5","volume-title":"Prostate-specific antigen (PSA) test","year":"2025"},{"key":"B6","volume-title":"FDA approves second PSMA-targeted PET imaging drug for men with prostate cancer","year":"2021"},{"key":"B7","doi-asserted-by":"publisher","DOI":"10.1016\/j.urology.2019.11.035","article-title":"Prostate cancer nonascitic peritoneal carcinomatosis after robot-assisted laparoscopic radical prostatectomy: 3 case reports and review of the literature","volume":"137","author":"Achard","year":"2020","journal-title":"Urology"},{"key":"B8","doi-asserted-by":"publisher","DOI":"10.1097\/RLU.0000000000005604","article-title":"Diffuse peritoneal carcinomatosis of prostate cancer unveiled by [89Zr]Zr-PSMA-617 PET\/CT","volume":"50","author":"Bastian","year":"2025","journal-title":"Clin\u00a0Nucl Med"},{"key":"B9","doi-asserted-by":"publisher","DOI":"10.1097\/RLU.0000000000002843","article-title":"Detection of unusual peritoneal metastases of prostate cancer with 68Ga-PSMA PET\/CT: case report","volume":"45","author":"Simsek","year":"2020","journal-title":"Clin Nucl Med"},{"key":"B10","doi-asserted-by":"publisher","DOI":"10.1097\/RLU.0000000000001703","article-title":"68Ga-PSMA PET\/MR-positive peritoneal metastasis in the falciform ligament in recurrent prostate cancer","volume":"42","author":"Kranzb\u00fchler","year":"2017","journal-title":"Clin Nucl Med"},{"key":"B11","doi-asserted-by":"publisher","DOI":"10.1080\/15384047.2018.1523849","article-title":"Next-generation sequencing of prostate cancer: genomic and pathway alterations, potential actionability patterns, and relative rate of use of clinical-grade testing","volume":"20","author":"Ikeda","year":"2019","journal-title":"Cancer Biol Ther"},{"key":"B12","doi-asserted-by":"publisher","DOI":"10.3322\/caac.21720","article-title":"Genetic testing in prostate cancer management: considerations informing primary care","volume":"72","author":"Giri","year":"2022","journal-title":"CA Cancer J Clin"},{"key":"B13","doi-asserted-by":"crossref","DOI":"10.1177\/108155899804600211","article-title":"Elevated carcinoembryonic antigen in patients with androgen-independent prostate cancer","volume":"46","author":"Feuer","year":"1998","journal-title":"J Investig Med"},{"key":"B14","doi-asserted-by":"crossref","first-page":"1264","DOI":"10.1002\/pros.24400","article-title":"Elevated serum CEA is associated with liver metastasis and distinctive circulating tumor DNA alterations in patients with castration-resistant prostate cancer","volume":"82","author":"Bray","year":"2022","journal-title":"Prostate"},{"key":"B15","doi-asserted-by":"crossref","DOI":"10.1186\/s40792-020-01074-7","article-title":"A rare case of PSA-negative metastasized prostate cancer to the stomach with serum CEA and CA19-9 elevation: a case report","volume":"6","author":"Shindo","year":"2020","journal-title":"Surg Case Rep"}],"container-title":["Frontiers in Oncology"],"original-title":[],"link":[{"URL":"https:\/\/www.frontiersin.org\/articles\/10.3389\/fonc.2026.1736081\/full","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2026,4,1]],"date-time":"2026-04-01T12:26:38Z","timestamp":1775046398000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.frontiersin.org\/articles\/10.3389\/fonc.2026.1736081\/full"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2026,4,1]]},"references-count":15,"alternative-id":["10.3389\/fonc.2026.1736081"],"URL":"https:\/\/doi.org\/10.3389\/fonc.2026.1736081","relation":{},"ISSN":["2234-943X"],"issn-type":[{"value":"2234-943X","type":"electronic"}],"subject":[],"published":{"date-parts":[[2026,4,1]]},"article-number":"1736081"}}