Impact of preexisting mental illness on breast cancer endocrine therapy adherence

Purpose Patients with estrogen receptor positive (ER+) breast cancer are often non-adherent to endocrine therapies, despite clear survival benefits. We utilized a nationally representative cancer cohort to examine the role of specific mental illnesses on endocrine therapy adherence. Methods Using the SEER-Medicare database, we included 21,894 women aged 68+ at their first surgically treated stage I-IV ER+ breast cancer during 2007–2013. All had continuous fee-for-service Medicare Parts A and B for 36+ months before, 18+ months after diagnosis, and continuous Part D for 4+ months before, 18+ after diagnosis. Mental illness was defined as occurring in the 36 months prior to cancer onset. We analyzed endocrine therapy adherence, initiation, and discontinuation using longitudinal linear and Cox regression models. Results Unipolar depression (11.0%), anxiety (9.5%), non-schizophrenia psychosis (4.6%), and dementias (4.6%) were the most prevalent diagnoses. Endocrine therapies were initiated by 80.0% of women. Among those with at least one year of use, 28.0% were non-adherent (< 0.80 adherence, mean = 0.84) and 25.7% discontinued. Patients with dementia or bipolar depression/psychotic/schizophrenia disorders had lower adjusted initiation probabilities by year one of follow-up, versus those without these diagnoses [0.74 95% CI (0.73–0.74) and 0.73 (0.72–0.73), respectively, reference 0.76 (0.76–0.77)]. Patients with substance use or anxiety disorders less frequently continued endocrine therapy for at least one year, after adjustment, [0.85 95% CI (0.85–0.86) and 0.88 (0.87–0.88), respectively, reference 0.90 (0.89–0.90)]. Patients with substance use disorders had 2.3% lower adherence rates ( p < 0.001). Conclusions Nearly one-quarter of female Medicare beneficiaries have diagnosed mental illness preceding invasive breast cancer. Those with certain mental illnesses have modestly reduced rates of initiation, adherence, and discontinuation and this may help define patients at higher risk of treatment abandonment. Overall, endocrine therapy adherence remains suboptimal, unnecessarily worsening recurrence and mortality risk.