LAUSR

Background Melanoma of unknown primary (MUP) is incompletely described on a population level. Objective We sought to characterize stage IV MUP in a population‐based cancer registry. Methods We developed a novel search algorithm to identify cases of stage IV MUP in the Surveillance, Epidemiology, and End Results 18 registries from 1973 to 2014. Cases of stage IV melanoma of known primary (MKP) served as a comparison group. Age‐standardized incidence rates, demographic characteristics, adjusted disease‐specific survival, and Cox proportional hazard models were calculated for MUP and MKP. Results A total of 322 stage IV MUP cases and 12,796 stage IV MKP cases were identified in Surveillance, Epidemiology, and End Results 18 registries from 1973 to 2014. The incidence of stage IV MUP is increasing, particularly for patients younger than 30 years of age. In multivariate analyses, age older than 50 and a lack of surgical treatment were negative prognostic factors for stage IV MUP. Relative survival, but not 5‐year adjusted disease‐specific survival, was higher for stage IV MUP than for MKP. Limitations Limitations include the retrospective study design and possible misclassification of MUP. Conclusions The incidence of stage IV MUP is increasing, and stage IV MUP shares similar prognostic factors with stage IV MKP, including age and surgical treatment.