Long Term Outcomes of Radiation (RT)-Monotherapy vs. Combined RT + Endocrine Therapy (RT+ET) in Low-Risk Early-Stage Breast Cancer Patients 70 Years or Older after Breast-Conserving Surgery (BCS)

Abstract

Combined Radiation and Endocrine Therapy (RT+ET) and ET Monotherapy are acceptable adjuvant strategies based on randomized trial data. Less is known about RT Monotherapy as a form of treatment de-escalation. Radiation delivery has become safer and more convenient. This study compares long-term outcomes in patients who opted for RT-Monotherapy versus combined RT+ET. This retrospective study included female patients from Swedish Cancer Institute breast cancer registry (Seattle, WA), aged ≥70 years with estrogen receptor-positive (ER+)/human epidermal growth factor receptor 2-negative (HER2-) T1N0 breast cancer treated with BCS between 1995 and 2015. Patients with positive surgical margins, multifocality, histologic grade 3 and LVI were excluded. Patients were classified into 4 groups: (1) RT+ET (reference); (2) RT Monotherapy; (3) ET Monotherapy; and (4) neither RT nor ET (NT). Choice of therapy was determined by the treating physician and the patient. The primary endpoints were local recurrence (LR), distant metastasis (DM), disease specific survival (DSS), overall survival (OS), and second breast cancer events (SBCEs). Competing risk models were used to estimate cumulative incidence at varying lengths of follow-up and sub distribution hazard ratios (HR) between treatment groups. Significance was set at P ≤ .05. We analyzed 496 patients with a median age of 76 years and median follow-up of 9.6 years. 62% (N = 307) of patients received combined RT+ET, 30% (N = 148) RT Monotherapy, 4% (N = 20) ET alone and 4% (N = 21) no therapy, NT group. Only the RT monotherapy and combined RT+ET groups were large enough for a meaningful statistical analysis. RT monotherapy conferred non-inferior disease control compared to combined RT+ET. Recurrence events were extremely low. Incidence of SBCEs was not significantly different between treatments. For this favorable patient population, RT monotherapy was associated with equivalent long-term outcomes compared to combined RT+ET therapy. Recurrence rates stayed extremely low with long-term FU. RT monotherapy may be a reasonable de-escalation option for this older low risk patient population. Further prospective trials are needed to help refine treatment strategies that allow for more choices in treatment de-escalation.