Small-cell neuroendocrine carcinoma of the descending colon in a young adult: a case report.

Neuroendocrine neoplasms (NENs) are a heterogeneous group of tumors arising from neuroendocrine cells with malignant potential. NENs of the colorectum are uncommon, and colonic neuroendocrine neoplasms (CNENs) are particularly rare. Here, we report the diagnostic and therapeutic course of a man in his 25-year-old with small-cell neuroendocrine carcinoma (SCNEC) of the descending colon. The patient presented with sudden, severe abdominal pain accompanied by nausea and vomiting. Computed tomography (CT) revealed circumferential wall thickening of the descending colon, blurring of the surrounding fat planes, and multiple small regional lymph nodes; contrast-enhanced CT demonstrated marked, homogeneous, and persistent enhancement. The initial imaging findings were easily confounded with inflammatory disease, and the patient declined further evaluation and was discharged. Nine months later, he was readmitted because of progressive intestinal obstruction. Imaging showed substantial progression of the lesion with extension to the sigmoid colon, accompanied by signs of extra-serosal invasion and peritoneal dissemination. Colonoscopy and histopathology confirmed SCNEC. The patient received ascites control, etoposide plus carboplatin chemotherapy, and supportive care, but the response was poor. This combination of young age and tumor location in the descending colon appears to be uncommon in the existing literature. The case is further distinguished by the presence of circumferential colonic wall thickening on imaging, a feature absent in prior reports. This case highlights that colonic SCNEC in young patients may present with atypical imaging features. Colonic NENs should be included in the differential diagnosis, and early endoscopic biopsy should be performed; definitive diagnosis relies on histopathology and immunohistochemistry to facilitate early recognition and standardized management.