INTRODUCTION: Adjuvant endocrine therapy improves recurrence-free and overall survival in primary breast cancer. However, not all patients complete their planned treatment, mostly because of side-effects. The aim of this study was to examine the adherence to adjuvant endocrine therapy in a cohort of primary breast cancer patients in Region Jönköping County, Sweden, after 3 and 5 years.

MATERIAL AND METHODS: The Swedish Breast Cancer Register was used to identify patients diagnosed with hormone receptor positive breast cancer in Region Jönköping County between 2009 and 2012. Adherence was evaluated based on data from the Swedish Prescribed Drug Register, and Medication Possession Ratio (MPR), defined as the days' supply of medication during the period from the first dispensing till the last dispensing in the time period (3 and 5 years), divided by number of days. Adherence was defined as MPR ≥80%. Regression analyses were used to identify subgroups associated with adherence; age, type of endocrine treatment, additional adjuvant therapy, and hospital responsible for the follow-up (Eksjö, Jönköping, and Värnamo).

RESULTS: We identified 634 patients who were recommended adjuvant endocrine therapy and to be able to estimate adherence after 3 and 5 years, 488 patients were included in the analysis. After 3 years of treatment, 91.2% of the patients (95% confidence interval (CI) 88.7-93.6; n = 445), were found to be adherent. The corresponding figure for the 271 patients who had completed 5 years of treatment was 91.5% (95% CI 88.2-94.8; n = 248). No subgroups (age, endocrine therapy, radio/chemotherapy, or hospital) were significantly associated with adherence in the multiple logistic regression analysis.

DISCUSSION: This study shows substantially higher adherence to adjuvant endocrine therapy than previously reported. Reasons for this could be differences in routines for therapy information and follow-up, but this needs to be further investigated.