Case Report: Prostate adenocarcinoma with peritoneal carcinomatosis and elevated CEA mimicking colorectal cancer: a diagnostic dilemma

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 + 4) 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 pain. 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.