Long-Term Adherence to Adjuvant Endocrine Therapy Following Various Radiotherapy Modalities in Hormone Receptor Positive Breast Cancer

Abstract

Adjuvant endocrine therapy (AET) is standard of care following breast-conserving surgery (BCS) for hormone receptor (HR) positive breast cancer. We compared the rates of long-term AET adherence after various radiation therapy (RT) modalities among patients with estrogen receptor (ER) and/or progesterone receptor (PR) positive breast cancer treated with BCS.Medical records from patients with stage 0 or stage 1, ER and/or PR positive breast cancer that received adjuvant RT between 2013 and 2015 at a single institution were retrospectively reviewed. We excluded patients with prior breast cancer or ductal carcinoma in situ (DCIS), HR-negative disease, primary tumors > 3cm, involved nodes, and those for whom AET was contraindicated. All patients received BCS followed by adjuvant RT via one of the following modalities: whole breast radiotherapy (WBI), accelerated partial breast irradiation (APBI) with either external beam radiation therapy (EBRT) or fractionated intracavitary high dose rate (HDR) brachytherapy, or single fraction HDR-brachytherapy intraoperative-radiation therapy (IORT) delivered at the time of lumpectomy, on a clinical trial (Precision Breast IORT). Only patients from the completed phase of the protocol were included. All patients were evaluated for up to five years of adherence to AET. Patient and treatment factors were evaluated via a Cox Regression analysis with backward selection of variables based upon P < 0.10 on univariate analysis (UVA) for inclusion in the final model. The Kaplan Meier method was used to model and further evaluate rates of AET adherence based on RT modality and histology as well as among the entire cohort.Ninety-seven patients were eligible for analysis. Thirty patients were treated with WBI, 41 with APBI, and 26 with IORT on clinical trial with a median follow up of 64.2, 72.0, and 58.6 months, respectively. Follow up schedule and evaluation of AET adherence was similar between all groups. For the entire cohort, AET adherence was approximately 67% at 2 years and 58% at 5 years. Among patients in the IORT clinical trial, adherence to AET was 53% at 2 years and 41% at 5 years. In the Cox regression model, histology (DCIS) and RT modality (IORT) were associated with decreased AET adherence at 5 years, with odds ratio (OR) 1.974 (CI: 1.018-3.828, P = 0.044) and OR 2.177 (CI: 1.132-4.186, P = 0.02), respectively.For early stage, HR positive breast cancer, the rate of AET adherence was approximately 58% at 5 years, consistent with prior reports. DCIS histology and receipt of IORT on clinical trial were associated with lower rates of adherence to AET at 5 years. Our findings suggest a potential need for further research aimed at addressing adherence to AET. Additionally, examination of the efficacy of more convenient RT interventions such as APBI and IORT in patients who do not receive AET is warranted.