Background Neuroendocrine tumours (NETs) can secrete bioactive amines into the bloodstream, causing carcinoid syndrome, with symptoms including flushing and diarrhoea. However, carcinoid syndrome frequency in the NET population has never been rigorously assessed, nor has its relationship to presenting clinicopathological characteristics. This analysis assessed the proportion of patients with NETs and carcinoid syndrome in the USA and associated clinical factors. Methods We identified patients (≥65 years of age) from the Surveillance, Epidemiology, and End Results-Medicare database, excluding those with pancreatic tumours or small-cell or large-cell lung cancer, as well as those without complete data. We assessed the incidence of patients with at least two insurance claims of flushing, diarrhoea, or carcinoid syndrome during the 3 months before and after NET diagnosis. We compared demographic and clinical characteristics between patients with and without carcinoid syndrome using χ2 tests. We used the Cochran-Armitage trend test to identify trends in carcinoid syndrome incidence and Cox regression to assess the relationship between carcinoid syndrome and survival. Findings Between April 1, 2000, and Dec 31, 2011, 9512 eligible patients were diagnosed with NETs, of whom 1786 (19%) had carcinoid syndrome. The number of patients with NETs and carcinoid syndrome increased from 50 (11%) of 465 patients in 2000 to 160 (19%) of 854 in 2011 (p<0·0001). The proportion of patients with carcinoid syndrome compared with those without did not differ significantly with respect to age at diagnosis (p=0·65), geographical region (p=0·054), or urban versus rural status (p=0·53). Patients with carcinoid syndrome were more frequently female than male (p=0·0003). Race was associated with a significant difference in the reported incidence of carcinoid syndrome (p<0·0001), as was tumour grade, stage, and primary tumour site (all p<0·0001). Patients with carcinoid syndrome had a shorter overall survival (median 5 years [95% CI 4·5–5·4]) than did those without carcinoid syndrome (5·6 years [5·4–5·9]; hazard ratio 1·102 [1·016–1·194]; p=0·019). Use of octreotide (p<0·0001) and chemotherapy (p=0·003) were more common in patients with carcinoid syndrome than in those without it, whereas surgery was used more frequently in patients without carcinoid syndrome (p=0·009); use of radiotherapy was not significantly associated with the presence of carcinoid syndrome at diagnosis (p=0·07). Interpretation This population-based analysis reveals that carcinoid syndrome is significantly associated with tumour grade, stage, and primary tumour site, and leads to shorter survival compared with those patients without carcinoid syndrome. An improved understanding of the heterogeneity of presenting symptoms among patients with NETs might permit more tailored assessment and management than at present and enable future research into the effect of carcinoid syndrome control on patient survival. Funding Ipsen.
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