Multiple sclerosis and cancer incidence: A Danish nationwide cohort study

Background: Introduction of disease modifying treatment may have increased the cancer incidence in multiple sclerosis patients. Our aim was to estimate the incidence of any cancer, malignant melanoma, nonmelanoma skin cancer, and female breast cancer, and cancer specific mortality in multiple sclerosis patients diagnosed in 1995-2015 i.e. after introduction of disease modifying treatment. Methods: Linking various Danish medical registers, we compared observed cancer incidence and cancer-specific mortality in multiple sclerosis patients versus expected based on general population rates. Results: Among 10,752 multiple sclerosis patients, we identified 5.76 incident cancers per 1,000 person-years. The standardized incidence ratio was 0.98 (95% confidence interval [CI], 0.90-1.06) for any cancer, 0.99 (95% CI, 0.84-1.15) for non-melanoma skin cancer, and 0.98 (95% CI, 0.81-1.18) for female breast cancer. For malignant melanoma, the standardized incidence ratio was 1.51 (95% CI, 1.13-1.98) for the entire period (1995-2015) but 1.16 (95% CI, 0.62-1.99) for 2005-2015. The overall mortality rate was 1.31 (95% CI, 1.09-1.53) per 1000 person-years with a standardized mortality ratio of 0.99 (95% CI, 0.83-1.17). Conclusion: In this nationwide study, multiple sclerosis patients did not have increased cancer incidence or increased cancer-specific mortality. We observed an increased risk of malignant melanoma mainly attributed to increased risk in the first part of our study period.