[Adjuvant endocrine therapy adherence in breast cancer patients diagnosed in Iceland 2013-2018]

Adjuvant endocrine therapy (tamoxifen/aromatase inhibitors) is used after surgery in hormone-receptor-positive breast cancer stage I-III. Administration for 5-10 years reduces recurrence and improves survival. Therapy adherence has not been assessed in Iceland. The main objective was to assess adherence to endocrine therapy among patients diagnosed with breast cancer in Iceland 2013-2018. Prognostic factors and the effect of adherence on prognosis were examined.

This retrospective, population-based cohort study obtained data from the Icelandic Cancer Registry, the Icelandic Prescription Medicine Register and medical records. Treatment adherence was defined as prescription refills for tamoxifen and/or aromatase inhibitors for five years or until the day of death/recurrence of breast cancer. Patients were considered adherent if prescriptions covered ≥80% of the treatment period. Descriptive statistics, logistic regression analysis, and Kaplan-Meier survival analysis were performed.

The study included 1,008 patients, 990 women and 18 men, with a median age of 62.0 years. Logistic regression identified chemotherapy before and/or after surgery as a positive predictor (OR 2.04; 95%CI 1.36-3.10; < 0.001) and initial treatment with tamoxifen as a negative prognostic factor (OR 0.68; 95%CI: 0.50-0.94; p=0.02). Significant association was observed between treatment adherence and overall survival (p=0.025) and disease-free survival (p=0.0075).

Among patients, 27.0% were non-adherent to treatment. Significant association was observed between survival and adherence. It is important to improve treatment adherence, for example through multidisciplinary approaches within the healthcare system and increased patient education.