The Role of Adjuvant Radiation Therapy in Treating Older Breast Cancer Patients With Low Adherence to Endocrine Therapy.

Background: Older patients with hormone receptor-positive, human epidermal growth factor receptor 2 (HER2)-negative early-stage breast cancer (HPEsBC) typically undergo breast-conserving surgery (BCS) followed by adjuvant radiation therapy (RT) and/or endocrine therapy (ET). Our study aimed to identify predictors of low ET adherence and evaluate the role of RT in modifying survival among patients with low ET adherence.

Methods: A retrospective analysis was performed using a US-based, electronic health record-derived, de-identified database. Patients aged ≥65 years with HPEsBC treated with BCS from 2011 to 2018 were included. Four adjuvant treatment groups were identified. Low ET adherence was defined as ET use for <80% of the 5-year post surgery follow-up period. Multinomial logistic regression was used to identify predictors of low adherence. Survival outcomes were assessed using hazard ratios (HRs) adjusted for covariates.

Results: A total of 1,488 patients were included in the study. Among patients receiving ET, 23% demonstrated low adherence. After adjustment for covariates, mortality was higher for RT alone (HR = 1.79, p = .011) and no adjuvant therapy (HR = 2.65, p < .001) compared with ET + RT. Predictors of low ET adherence included increasing age (odds ratio [OR] = 1.06, p < .010) and treatment at an academic practice (OR = 2.58, p < .001). A 10% decline in ET adherence was associated with increased mortality (HR = 1.17, p < .001). An interaction analysis revealed no differential effect of RT in the context of ET adherence.

Conclusion: Low ET adherence occurred in approximately one-quarter of patients and was associated with advancing age and treatment at academic centers. Reduced ET adherence was linked to significantly increased mortality. Further investigation into the role of RT in patients with low ET adherence is warranted.