Ten-Year Mortality after a Breast Cancer Diagnosis in Women with Severe Mental Illness: A Danish Population-Based Cohort Study

Background
Breast cancer is the leading cause of cancer death in women worldwide. Nevertheless, it is unknown whether higher mortality after breast cancer contributes to the life-expectancy gap of 15 years in women with severe mental illness (SMI).
Methods
We estimated all-cause mortality rate ratios (MRRs) of women with SMI, women with breast cancer and women with both disorders compared to women with neither disorder using data from nationwide registers in Denmark for 1980–2012.
Results
The cohort included 2.7 million women, hereof 31,421 women with SMI (12,852 deaths), 104,342 with breast cancer (52,732 deaths), and 1,106 with SMI and breast cancer (656 deaths). Compared to women with neither disorder, the mortality was 118% higher for women with SMI (MRR: 2.18, 95% confidence interval (CI): 2.14–2.22), 144% higher for women with breast cancer (MRR: 2.44, 95% CI: 2.42–2.47) and 327% higher for women with SMI and breast cancer (MRR: 4.27, 95% CI: 3.98–4.57). Among women with both disorders, 15% of deaths could be attributed to interaction. In a sub-cohort of women with breast cancer, the ten-year all-cause-mortality was 59% higher after taking tumor stage into account (MRR: 1.59, 95% CI: 1.47–1.72) for women with versus without SMI.
Conclusions
The mortality among women with SMI and breast cancer was markedly increased. More information is needed to determine which factors might explain this excess mortality, such as differences between women with and without SMI in access to diagnostics, provision of care for breast cancer or physical comorbidity, health-seeking-behavior, and adherence to treatment.