Supplementary Material for: A Real-World, Population-Based Study for the Incidence and Outcomes of Neuroendocrine Neoplasms of Unknown Primary

<b><i>Objective:</i></b> The aim of the study was to provide a real-world, population-based assessment of the incidence and outcomes of neuroendocrine neoplasms (NENs) of unknown primary. <b><i>Methods:</i></b> Surveillance, Epidemiology, and End Results registry was accessed, and cases with NENs of unknown primary were reviewed. Rates of NENs diagnosis 1975–2017 according to primary tumor site were also reviewed. Survival outcomes of patients with NENs of unknown primary compared to metastatic NENs with known primary were determined through Kaplan-Meier estimates and multivariable Cox regression analysis. Overall and cancer-specific survival analyses were stratified by primary site and histology (neuroendocrine tumor vs. neuroendocrine carcinoma). <b><i>Results:</i></b> A total of 3,550 cases (7%) were diagnosed with NENs of unknown primary within the study duration. The annual percent change for NENs of unknown primary was 3.4 (95% CI: 2.6–4.2). Within the cohort of metastatic neuroendocrine tumor patients (carcinoid tumor histology), the following factors were associated with a lower risk of death; younger age (HR: 0.477; 95% CI: 0.443–0.513), female sex (HR: 0.922; 95% CI: 0.860–0.989), and small intestinal primary (HR for unknown primary vs. small intestinal primary: 1.532; 95% CI: 1.408–1.668). Within the cohort of metastatic neuroendocrine carcinoma, the following factors were associated with a lower risk of death in this cohort; younger age (HR: 0.646; 95% CI: 0.612–0.681), female sex (HR: 0.843; 95% CI: 0.801–0.888), and small intestinal primary (HR for unknown primary vs. small intestinal primary: 2.961; 95% CI: 2.586–3.391). <b><i>Conclusions:</i></b> The diagnosis of NENs of unknown primary has increased across the past 4 decades. Outcomes of individuals with metastatic small intestinal NENs seem to be better than those with metastatic NENs of unknown primary (for both carcinoid tumors and neuroendocrine carcinomas).