Outcomes of carcinoid syndrome in small versus large intestinal neuroendocrine tumors.

620 Background: Carcinoid syndrome (CS) most commonly arises from small intestinal neuroendocrine tumors (NETs) due to their high propensity for hepatic metastases, while large intestinal NETs rarely produce CS and usually represent more advanced disease when they do. Whether outcomes differ between small and large intestinal NETs complicated by CS remains poorly defined. We compared clinical characteristics and hospitalization outcomes in patients with CS due to small versus large intestinal NETs using a national dataset. Methods: We queried the National Inpatient Sample (2016–2022) to identify adult hospitalizations with a diagnosis of carcinoid syndrome and malignant NETs of the small or large intestine. The primary outcome was inpatient mortality. Secondary outcomes included length of stay (LOS), hospitalization cost, need for mechanical ventilation, venous thromboembolism, hypovolemic shock, arrhythmias, and cardiac valvular disease. Multivariable logistic regression was performed to assess mortality risk adjusting for age, sex, race, and comorbidity burden. Results: A total of 2, 874 hospitalizations with CS and intestinal NETs were identified; 2, 519 (87. 6%) were small intestine and 355 (12. 4%) were large intestine. Patients with large intestinal NETs had a higher comorbidity burden (mean CCI 3. 2 vs. 2. 3, p=0. 02) but similar age (66. 1 vs. 65. 9 years, p=0. 9) and sex distribution (male 56. 3% vs. 49. 6%, p=0. 2) compared with small intestine NETs. Overall inpatient mortality was 6. 2% (n=179). Mortality was significantly higher in large intestinal NETs compared with small intestinal NETs (12. 6% vs. 5. 3%, p=0. 01). On multivariable analysis, large intestinal NETs with CS had a 156% increased odds of inpatient death compared to small intestinal NETs (adjusted OR 2. 56, 95% CI, p=0. 026). LOS (7. 2 vs. 7. 4 days, p=0. 8) and mean hospitalization costs (92, 980 vs. 108, 034, p=0. 4) were similar between groups. No significant differences were observed in mechanical ventilation, venous thromboembolism, atrial fibrillation/flutter, or hypovolemic shock. No cases of carcinoid heart syndrome, tricuspid stenosis, or pulmonary stenosis were identified. Conclusions: In this national cohort, carcinoid syndrome arising from large intestinal NETs was associated with more than a twofold higher risk of inpatient mortality compared with small intestinal NETs, despite similar demographics and resource utilization. These findings suggest important biological or clinical differences in outcomes by tumor site and highlight the need for further study of risk factors and management strategies in this rare population.