Primary Neuroendocrine Carcinoma of the Ileum With Markedly Elevated Carcinoembryonic Antigen (CEA) Levels: A Case Report.

Neuroendocrine carcinomas (NECs) are rare and highly malignant tumors with a generally poor prognosis. Carcinoembryonic antigen (CEA) is often associated with adenocarcinoma, but its significant elevation in NEC cases is unusual. A 69-year-old man was admitted to our hospital in January 2016 due to syncope induced by anemia. The patient had a hemoglobin level of 8.0 g/dL and an ileocecal mass causing small bowel obstruction on computed tomography. His CEA level was markedly elevated at 3625.4 ng/mL. A colonoscopy revealed a neoplastic lesion in the terminal ileum, leading to an emergency ileocecal resection. Pathology confirmed a NEC, positive for synaptophysin and CEA, with a Ki-67 index of 30%. The patient was diagnosed with stage IIIb NEC (pT3N2M0). A postoperative increase in CEA to 4124.6 ng/mL and metastases in the right lung and multiple lymph nodes were detected. Initial chemotherapy with irinotecan, cisplatin (IP), and octreotide acetate proved ineffective. Subsequent octreoscans showed disease progression. Switching to everolimus as second-line therapy temporarily decreased CEA levels and tumor size, but the disease progressed with cervical lymph node involvement. The patient underwent palliative radiotherapy but succumbed to disease progression in May 2018, with a final CEA level of 36,643 ng/mL. Necropsy of the cervical lymph nodes was consistent with the original surgical findings. This case highlights the aggressive nature and challenging management of NEC with significantly elevated CEA levels.