(SLA-006) Patterns of endocrine therapy adherence and the risk of breast cancer recurrence

Background: Five years of endocrine therapy (ET) is a key treatment for approximately 70% of breast cancer patients, yet adherence remains suboptimal. Identifying longitudinal patterns of ET adherence may help identify women who could benefit from adherence-support interventions and those who may be suitable candidates for extended ET beyond five years.

Objectives: To explore the relationship between patterns of adherence to the guideline-recommended five years of ET and breast cancer recurrence among patients who remained recurrence-free five years post-diagnosis.

Methods: We identified all women aged 18 to 70 diagnosed with stage I–III estrogen receptor-positive breast cancer who received ET and were registered in the Danish Breast Cancer Group (DBCG) clinical database (2002–2011). We further restricted the population to patients alive and recurrence-free five-years from diagnosis. We used DBCG treatment follow-up data to create supply diaries of ET adherence and employed group-based trajectory modeling to capture longitudinal adherence patterns in the first five years after breast cancer diagnosis. We then followed all patients from five years after diagnosis until DBCG-identified recurrence, mortality, another primary malignancy, or 10 years of follow-up. Hazard ratios (HRs) and 95% confidence intervals (CIs) were estimated using Cox proportional hazards modeling, adjusting for relevant clinical and demographic factors.

Results: Among 15,289 five-year breast cancer survivors, we identified four distinct patterns of ET adherence in the five years following diagnosis—total adherence (maintaining 100% adherence, n=5,043; 33%), high adherence (maintaining ~90% adherence, n=8,297; 54%), gradual discontinuers (declining to 0% adherence within four years, n=1,034; 6.7%), and rapid discontinuers (declining to 0% adherence within two years, n=915; 6.0%). Compared with women with total adherence, the estimated rate of breast cancer recurrence in the five years following ET treatment was increased among those with high adherence (HR: 1.09, 95% CI: 0.93, 1.29), gradual discontinuers (HR: 1.58, 95% CI: 1.14, 2.17), and rapid discontinuers (HR: 2.34, 95% CI: 1.71, 3.2).

Conclusions: Patients with declining adherence trajectories exhibited a higher rate of recurrence after completing five years of ET. While these findings suggest that adherence may influence recurrence risk even after treatment ends, the results should be interpreted cautiously due to potential bias, including healthy adherer bias. Future analyses will incorporate a late recurrence algorithm and apply methods to better account for such biases.