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There is an increased risk of second primary cancers (SPCs) after neuroendocrine tumor (NET) diagnosis. The clinical significance of SPCs in this population is unknown. The purpose of this study was to evaluate the association between SPCs after NET diagnosis and survival. We performed a population-based, retrospective cohort study of NET patients (gastrointestinal, pancreatic, or lung primary) from 2000 to 2016 using the Surveillance, Epidemiology, and End Results database. Cox regression models assessed the association between SPCs and NET-specific (NET-SS), cancer-specific (CSS), and overall survival (OS). Of 58,553 NET patients, 7.9% experienced an SPC. SPCs were associated with worse OS (hazard ratio (HR) 2.14, 95% CI 1.94-2.36) and CSS (HR 2.31, 95% CI 2.06-2.59) with no difference in NET-SS (HR 1.04, 95% CI 0.87-1.23). Stratified analyses by histologic grade showed similar results for well and moderately differentiated NETs, but no difference in OS or CSS for poorly differentiated NETs (P > 0.05). In stratified analyses by NET site, SPCs were associated with worse OS (HR 3.41, 95% CI 3.01-3.87) and CSS (HR 4.96, 95% CI 4.28-5.74) in gastrointestinal NETs and worse OS (HR 1.25, 95% CI 1.03-1.52) with no difference in CSS (HR 1.08, 95% CI 0.85-1.36) in lung NETs. SPCs were not associated with a difference in OS or CSS in pancreatic NETs (P > 0.05). In conclusion, SPCs after NETs were associated with inferior OS and CSS compared to no SPC but were not associated with NET-SS. These data highlight the need for long-term follow-up in NETs to include the detection of SPCs to ensure early diagnosis and timely management. [ABSTRACT FROM AUTHOR]