Objective — We tested the hypotheses that overall survival and cancer-specific survival after breast cancer diagnosis would be lower in persons with multiple sclerosis (MS) as compared to persons without MS using a retrospective matched cohort design.
Methods — We applied a validated case definition to population-based administrative data in Manitoba and Ontario, Canada to identify female MS cases. We linked the MS cohorts to cancer registries to identify women with breast cancer. Then we selected 4 breast cancer controls without MS matched on birth year, cancer diagnosis year and region. We compared all-cause survival between cohorts using Cox proportional hazards regression adjusting for age at cancer diagnosis, cancer diagnosis period, income quintile, region, and Elixhauser comorbidity score. We compared cancer-specific survival between cohorts using a multivariable cause-specific hazards model. We pooled findings between provinces using meta-analysis.
Results — We included 779 MS cases and 3116 controls with breast cancer. Most subjects with stage data (1976/2822, 70.0%) were diagnosed with stage I or II breast cancer, and the mean (SD) age at diagnosis was 57.8 (10.7) years. After adjustment for covariates, MS was associated with a 28% increased hazard for all-cause mortality (hazard ratio [HR] 1.28; 95%CI: 1.08-1.53), but was not associated with altered cancer-specific survival (HR 0.98; 95%CI: 0.65-1.46).
Conclusion — Women with MS have lower all-cause survival after breast cancer diagnosis than women without MS. Future studies should confirm these findings in other populations and identify MS-specific factors associated with worse prognosis.