Delays in initiating adjuvant endocrine therapy (AET) are a cause for concern among women with breast cancer and clinicians, but the impact of delayed AET on overall survival (OS) is unclear. This study seeks to describe the relationship between delayed AET and OS.

Retrospective cohort study of women with stage II and III hormone receptor positive, human epidermal receptor 2 negative, invasive breast cancer, identified from the National Cancer Database. The primary exposure delayed AET, was defined as initiation of AET more than 12 months after breast cancer diagnosis. Using logistic regression, we examined predictors of delayed AET. The survival analysis with Cox proportional hazards regression adjusted for patient, tumor, and treatment characteristics.

Among the 391,594 included women, 12,162 (3.1%) had delayed AET. Predictors of delayed AET included Black race (adjusted odds ratio [aOR] = 1.61, 95% confidence interval [CI] 1.52-1.70) or Hispanic ethnicity (aOR = 1.25, 95% CI 1.16-1.35) vs white race, Medicare (aOR = 1.13, 95% CI 1.06-1.20) or Medicaid (aOR = 1.41, 95% CI 1.32-1.50) versus private insurance, and cancer stage III (aOR = 1.24, 95% CI 1.19-1.30) vs stage II. With median follow-up of 67.4 months, 67,335 (17.2%) patients died. Delayed AET had no statistically significant effect on the hazard of death (adjusted hazards ratio = 1.01; 95% CI 0.96-1.06) compared to initiation within 12 months of diagnosis.

This study suggests that there may be no adverse impact on survival if initiation of AET occurs 12 to 24 months after initial diagnosis compared to within 12 months of diagnosis as currently recommended.