S4092 A Rare Case of Metastatic Small Cell Neuroendocrine Carcinoma of the Rectum

Introduction: Small cell neuroendocrine carcinoma (SCNEC) of the gastrointestinal tract is a rare and highly aggressive malignancy characterized by rapid progression and early metastasis. Unlike the more common adenocarcinomas of the colon and rectum, neuroendocrine tumors may present with atypical symptoms and can be diagnostically challenging due to their heterogeneous pathology. Here, we describe the case of a middle-aged woman whose initial nonspecific presentation belied an advanced and aggressive malignancy, ultimately confirmed as SCNEC. Case Description/Methods: A 45-year-old woman with no significant past medical history presented with a 3-week history of constipation and rectal bleeding. She denied weight loss, urinary symptoms, or a family history of cancer. Labs, complete blood count showed thrombocytosis (platelets 449 K/µL). Computed tomography abdomen revealed an 8.1 cm pelvic mass abutting the rectum, along with hepatic lesions, peritoneal nodules, a sclerotic iliac lesion, and a left hydrosalpinx, suggestive of metastatic disease. Magnetic resonance imaging pelvis demonstrated a circumferential rectal mass invading mesorectal fat and adjacent lymphadenopathy. Colonoscopy revealed an ulcerated mass from 6 to 12.5 cm from the anal verge. Initial biopsies showed only mucosal fragments with focal atypia. Positron emission tomography scan confirmed hypermetabolic activity in the pelvis, bones, upper abdomen, and adnexa. Flexible sigmoidoscopy with deeper biopsy was performed, which confirmed the diagnosis of SCNEC. The patient was started on systemic etoposide and cisplatin. She was deemed ineligible for surgery due to extensive metastases, and palliative radiation was considered. Discussion: This case highlights the aggressive nature of rectal SCNEC and the importance of high clinical suspicion when faced with atypical gastrointestinal presentations. The National Comprehensive Cancer Network and the European Neuroendocrine Tumor Society support platinum-based chemotherapy as first-line treatment for metastatic high-grade NECs, as was initiated in this case. Surgery is reserved for localized disease; hence, it is not pursued here. Prognosis for metastatic SCNEC remains poor, with a median survival of 10–16 months. Early diagnosis and accurate histopathology are critical to initiating appropriate therapy and optimizing outcomes.