Adherence to adjuvant endocrine therapy after breast cancer in Sweden - a nationwide cohort study in 1-, 3- and 5-year survivors with a focus on regional differences.

Background and purpose: Adjuvant endocrine treatment (AET) is crucial in early oestrogen receptor (ER)-positive breast cancer (BC), providing reduced recurrence rate and increased overall survival. The aim of this study was to estimate AET adherence rates by age at diagnosis and region in Sweden.

Patients and methods: In total, 10,422 women diagnosed with ER-positive BC in 2008-2010 were identified in the Swedish National BC Registry. Information on prescriptions and dispensation of AET was gathered through record linkage to the Swedish Prescription Registry. 1, 3- and 5-year medication possession ratios (MPRs) were calculated. Good adherence was set as MPR ≥ 80%.

Results: The 1-, 3- and 5-year AET age-adjusted adherence rates were 94.4, 87.6 and 81.6%, respectively. The 1-, 3- and 5- year adherence rate was significantly highest in the South region (96.2, 90.5 and 86.2%). Regions with an oncologic clinic had higher adherence rate than regions without, 82.8% versus 75.5% at 5-year FU. Women at age 40-64 years (95.6, 89.9 and 84.1%) and 65-74 years at diagnosis (95.7, 89.5 and 84.6%) had significantly higher adherence rate than women ≥ 75 years at diagnosis (89.1, 79.2 and 68.3%).

Interpretations: Despite guidelines being national, there were significant differences in adherence between regions in Sweden. As the largest differences were between age groups invited and not invited to mammography screening intervention should focus on women < 40 and ≥ 75 years at diagnosis. Further studies are needed to find strategies to increase overall adherence to AET in early BC.