Survival in patients diagnosed with melanoma in situ compared to the general population. A Swedish population-based matched cohort study.

Background: The incidence of melanoma in situ (MIS) is increasing even more rapidly than the incidence of cutaneous malignant melanoma (CMM). No previous studies have in detail investigated the survival in individuals diagnosed with MIS compared to the general population. Methods: This population-based study included individuals with MIS diagnosed in Sweden between 2001 and 2010 and randomly selected MIS-free comparators matched on age, sex and county of residence. Exclusion criterion was a previous CMM. Data on socioeconomic status (SES) including educational level, income and marital status, comorbidity and cause of death were obtained from population-based registers. Overall survival (OS) was estimated by the Kaplan-Meier method. The mortality risk adjusted for SES and comorbidity was assessed by multivariable Cox regression analyses. Findings: The survival analyses included 7963 cases and 39,662 comparators. Median age at MIS diagnosis were 63 (IQR 50-75) and 67 (IQR 57-76) years in women and men respectively. Median follow-up time was 120 months (IQR 102-152 months). In individuals with MIS, the ten-year OS was 77% (95% CI 0.76-0.78) compared to 72% (95% CI 0.72-0.73) in comparators. The MIS patients had a higher SES and lower comorbidity burden than the comparators. In a fully adjusted multivariable analysis, including 7772 cases and 38,103 comparators, the mortality was significantly lower in women with MIS (HR 0.88, 95% CI 0.82-0.94) compared to the background population. The corresponding estimate in men was HR 0.94 (95% CI 0.88-1.0). The risk of melanoma-related deaths during the study period was ten-fold higher in MIS patients. Interpretation: Despite being at increased risk of developing CMM, MIS patients had a better OS compared to their matched comparators from the background population, findings which could not fully be explained by differences in SES and comorbidity. Our results are reassuring and should be communicated to patients who have been diagnosed with MIS. Funding: Stiftelsen Onkologiska Klinikens i Uppsala Forskningsfond, Mats and Stefan Paulsson Trust, Medicon Village, Lund and Uppsala University Hospital (ALF).