Evaluating the Quality of Real-World Data on Adherence to Oral Endocrine Therapy in Breast Cancer Patients: How Real Is Real-World Data?

Purpose: The aim of this study was to compare estimates of adherence to oral endocrine therapy (OET) based on real-world data (RWD) and on clinical evaluation in people diagnosed with breast cancer in the public healthcare system in Catalonia (Spain).

Methods: We conducted two retrospective cohort studies. Cohort 1 (RWD) consisted of women diagnosed with breast cancer in 2021 in the public healthcare system of Catalonia (Spain). Sources of RWD were the pharmacy billing register, hospital discharge records, and the Catalan health division's central insurance registry. Nonadherence was defined as below 80% adherence in the first year of treatment. Data for cohort 2 came from two population-based cancer registries in Girona and Tarragona (Catalonia), with diagnoses from 2007 to 2011. We evaluated the impact of variables missing from RWD, such as stage and hormonal status. Analyses were performed using a chi-square test and logistic regression, with results stratified by age group and drug type.

Results: Nonadherence at one year was 10.9% in cohort 1 and 11.3% in cohort 2. When we reviewed the medical records of a selection of nonadherent women from cohort 1, we found only 59.4% had documented treatment interruptions. Reasons for interruptions in the patients from RWD cohort included adverse effects (48.8%), patient decision (40.0%), medical reasons (29.4%), and other clinical causes (14.7%). Women aged under 50 years and those receiving tamoxifen or a sequential regimen had lower adherence. Determinants associated with nonadherence were similar in both approaches used.

Conclusions: This study confirms the validity of estimating adherence with RWD from the Spanish national health system, although when combined with reviewing medical records, this may provide more reliable and higher-quality data. The RWD method provides valuable evidence to help oncologists discuss adherence with their patients.