ESTRO 2025 - Abstract Book

S425

Clinical - Breast

ESTRO 2025

osteoarthritis (OA), and the timing of OA symptom onset in influencing AHT adherence and overall survival in a nationwide cohort of breast cancer patients in Korea.

Material/Methods: This nationwide retrospective cohort study utilized data from the Korean National Health Insurance Service, including 33,142 women diagnosed with primary invasive breast cancer between 2011 and 2015. Group-based trajectory modeling (GBTM) was used to classify distinct AHT adherence patterns over a five-year period based on the proportion of days covered (PDC). We employed competing risk regression and Cox proportional hazards models to examine how socioeconomic factors, pre-existing OA, the time to first OA-related NSAID (nonsteroidal anti-inflammatory drug) prescription, and other clinical variables influence AHT discontinuation and survival outcomes. Variables such as type of insurance coverage and geographic location were also analyzed. Results: GBTM analysis revealed two primary adherence groups: a high adherence group (83.4%) and a low adherence group (16.6%). The low adherence group experienced a more rapid decline in PDC over time, which was associated with a significantly higher risk of AHT discontinuation (SHR: 14.06, 95% CI: 12.50–14.96, p<0.001) and mortality (HR: 3.56, 95% CI: 3.09–4.09, p<0.001). Patients with a longer history of OA prior to AHT initiation (p = 0.001) and those who used NSAIDs before starting AHT (p<0.001) were more likely to discontinue treatment early. Interestingly, a longer time to the first OA-related NSAID prescription after AHT initiation was associated with a lower risk of discontinuation (p<0.001). Furthermore, patients with Medical Aid/Veteran insurance (OR: 0.60, 95% CI: 0.53–0.67, p<0.001) and those residing in non-capital areas (OR: 0.74, 95% CI: 0.69–0.79, p<0.001) were less likely to belong to the high adherence group, suggesting that socioeconomic status plays a crucial role in adherence patterns.