Factors associated with endocrine therapy adherence among post-menopausal women treated for early-stage breast cancer in Ontario, Canada

Purpose — Adherence to adjuvant endocrine therapy among post-menopausal breast cancer patients is an important survivorship care issue. We explored factors associated with endocrine therapy adherence and survival in a large real-world population-based study.

Methods — We used health administrative databases to follow women (aged ≥ 66 years) who were diagnosed with breast cancer and started on adjuvant endocrine therapy from 2005 to 2010. Adherence was measured by medical possession ratio (MPR) and characterized as low (< 39% MPR), intermediate (40-79% MPR), or high (≥ 80% MPR) over a 5-year period. We investigated factors associated with adherence using a multinomial logistic regression model. Factors associated with all-cause mortality (5 years after starting endocrine therapy) were investigated using a multivariable Cox proportional hazards model.

Results — We identified 5692 eligible patients starting adjuvant endocrine therapy who had low, intermediate, and high adherence rates of 13% (n = 749), 13% (n = 733), and 74% (n = 4210), respectively. Lower rates of adherence were associated with increased age [low vs. high adherence: odds ratio (OR) 1.03, 95% CI 1.02-1.05 (per year); intermediate vs. high adherence: OR 1.02, 95% CI 1.01-1.04 (per year)]. High adherence was associated with previous use of adjuvant chemotherapy (low versus high adherence OR 0.42, 95% CI 0.30-0.59) and short-term follow-up with a medical oncologist within 4 months of starting endocrine therapy (low versus high adherence OR 0.83, 95% CI 0.69-0.99). Unadjusted analysis showed increased survival among patients with high endocrine therapy adherence. However, an independent association was no longer clearly detected after controlling for confounders.

Conclusion — Interventions to improve adjuvant endocrine therapy adherence are warranted. Non-adherence may be a more significant issue among elderly patients. Short-term follow-up visit by a patient's medical oncologist after starting endocrine therapy may help to improve compliance.