Risk of cancer among multiple sclerosis patients, siblings, and population controls: A prospective cohort study.

Background: Risk of cancer in multiple sclerosis (MS) patients compared to their siblings is unknown.
Objective: The objective was to prospectively investigate the risk of cancer among MS patients compared to siblings without MS and to population controls.
Methods: We retrieved data on MS patients born between 1930 and 1979 from the Norwegian Multiple Sclerosis Registry and population studies and on cancer diagnosis from the Cancer Registry of Norway. We used adjusted Cox proportional hazard regression to estimate cancer risk among 6883 MS patients, 8918 siblings without MS, and 37,919 population controls.
Results: During 65 years of follow-up, cancer risk among MS patients was higher than that among population controls (hazard ratio (HR) = 1.14, 95% confidence interval (CI): 1.05-1.23) in respiratory organs (HR = 1.66, 95% CI: 1.26-2.19), urinary organs (HR = 1.51, 95% CI: 1.12-2.04), and the central nervous system (HR = 1.52, 95% CI: 1.11-2. 09). Siblings had higher risk of hematological cancers compared with MS patients (HR = 1.82, 95% CI: 1.21-2.73) and population controls (HR = 1.72, 95% CI: 1.36-2.18).
Conclusion: MS patients were associated with increased risk of cancer compared to population controls. Siblings had increased risk of hematological cancer. This indicates that MS and hematological cancer could share a common etiology.