Abstract P1-13-03: Radiotherapy vs. endocrine therapy for hormone receptor positive early stage breast cancer accounting for endocrine therapy adherence
Authors: Seol Seung Won , Pflederer Travis , Weller Lauren , Goodman Chelain , Donnelly Eric D , Hayes John P , Strauss Jonathan B 
Published: March 2022

Abstract
Purpose/Objective(s):. Several trials have compared endocrine therapy (ET) alone vs. ET and radiotherapy (RT) in low risk early stage estrogen receptor (ER)-positive breast cancer and have shown that the addition of RT to ET modestly reduces the rate of loco-regional recurrence (LRR) without altering survival. ET may also yield modest reductions in contralateral breast events. ET alone has thus become a standard strategy in select patients. Data evaluating the efficacy of RT alone are scarce. However, the toxicity of ET is not trivial and its effectiveness is dependent upon treatment adherence. As RT courses become ever shorter and safer, a re-evaluation of the strategy of RT alone is warranted. This analysis reports on the treatment outcomes of ER-positive early stage breast cancer treated with adjuvant RT or ET alone at a multi-hospital health system. We hypothesized that adjuvant RT alone would yield loco-regional control comparable to that of ET alone and that ET adherence would influence outcomes. Materials/Methods:. The shared electronic health record of multiple facilities within a university health system was queried to identify women with pathologic T1-2 N0 breast cancer treated with breast-conserving surgery between 2007 and 2016. Data collection included demographic features, tumor characteristics, treatment type, and evidence of LRR. Analysis was restricted to those patients with ER positive disease who underwent either RT alone or ET alone. Five-year LRR were compared across groups using the Kaplan-Meier method. Results:. 240 patients met inclusion criteria. Of these, 86 were treated with RT alone and 154 with ET alone. Patient, tumor and ET variables are listed in the table below. ET adherence and duration information were available for 135 out of 154 patients who received ET alone. Patients who completed 5 years of ET or whose ET was discontinued due to the disease recurrence or death were categorized as ET-adherence (ET-A). Patients who discontinued ET before reaching 5 years due to other reasons were categorized as ET-non adherence (ET-NA). Median ET duration in ET-A was 60 months (range, 15-101 months) and in ET-NA was 13 months (range, 1-54 months). The 5-year rates of LRR for RT alone vs. ET alone were 4.1 % (95% CI 0-8.8) vs. 5.0 % (95% CI 1.1-8.9). The 5-year rates of LRR for ET-A vs. ET-NA were 2.0 % (95% CI 0-4.7) vs. 16.4 % (95% CI 1.1-31.7). While the difference in the LRR rates between RT alone vs. ET-A was not statistically significant (p= .481), ET-NA was associated with significantly higher LRR rate compared to RT alone or ET-A (p&lt;.05). The rate of ET non-adherent rate was 21.4%.
Conclusion:. RT yielded LRR equivalent to overall ET and superior to ET non-adherence. For patients with early stage ER+ breast cancer, a prospective randomized trial comparing RT to ET and evaluating quality of life (QoL) is needed. In light of emerging data, a five-day course of RT can be considered as an alternative to five years of ET, and RT should be strongly considered if ET adherence is in doubt.
Table 1.Patient, tumor and treatment characteristicsRT alone (n=86)ET alone (n=154)Median follow-up (months)50.6 (1-135)48 (3-114)Age (median)63 (39-91)74 (50-87)StageT176 (88.4%)142 (92.2%)T210 (11.6%)12 (7.8%)HistologyGrade 38 (9.3%)9 (5.8%)+LVI5 (5.8%)3 (2.0%)Endocrine Therapy (ET)ET duration (median, months)-57 (1-101)ET-Adherent-102 (66.2%)ET-Non adherent-33 (21.4%)ET duration not available-19 (12.3%)
Citation Format: Seung Won Seol, Travis Pflederer, Lauren Weller, Chelain Goodman, Eric D Donnelly, John P Hayes, Jonathan B Strauss. Radiotherapy vs. endocrine therapy for hormone receptor positive early stage breast cancer accounting for endocrine therapy adherence [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P1-13-03.