2940 - Outcomes of Adjuvant Ultra-Hypofractionated Radiation (UHFRT) in Older Women with Low Risk, Early-Stage Breast Cancer Who Are Not Compliant with Endocrine Therapy

Purpose/Objective(s): Motivated patients with low-risk early-stage breast cancer, undergoing breast-conserving surgery (BCS), may omit radiotherapy (RT) so long as they proceed with endocrine therapy (ET). Emerging evidence suggests that RT alone may render better outcomes compared to ET alone. This study aims to investigate the outcomes of ultra-hypofractionated radiation therapy (UHFRT) in patients with low-risk breast cancer who are potential candidates for omission of RT but who receive UHFRT because they do not comply with endocrine therapy (ET).

Materials/Methods: After IRB approval, outcomes of women treated at a multi-site institution with BCS and adjuvant 5-fx UHFRT: with either whole breast irradiation (WBI) 26Gy/5fx or accelerated partial breast irradiation (APBI) 26-30Gy/5fx between Jan 2020 and July 2024. Inclusion criteria were age= 65 years, Grade 1 or 2, T1-T2 up to 3 cm invasive breast cancer, axillary node-negative, clear margins, hormone receptor-positive, HER-2 negative. Patients with poor compliance to or omission of ET were identified and compared to those compliant with ET. Toxicity, cosmetic outcomes, and ipsilateral breast tumor recurrence (IBTR) were analyzed. Descriptive statistics, Wilcoxon rank sum test, Chi-square test and logistic regression analysis were performed.

Results: A total of 261 women met low-risk inclusion criteria and were treated with UHFRT, 128 (49%) had WBI and 133 (51%) APBI. While 192 (74%) of women complied with ET, 69 (26%) did not. There was 100% compliance with UHFRT. Median age was 75 years (range, 65 - 94 years) Median tumor size was 0.9 cm (range, 0.1 – 3.0 cm). At a median follow up of 21 months (range, 1 - 37 months), there was no IBTR in the entire cohort. Median age was older in the UHFRT alone cohort (77 years, range 65 -94 years) than in UHFRT+ET (75 years, range 65- 84 years) (p=0.004). Grade 2+ acute toxicities, late toxicities, and cosmetic outcomes (induration) were similar between UHFRT alone and UHFRT+ET (0% vs. 1.6%, 3% vs. 1%, and 1.5% vs. 1.6%, respectively; p=0.30, 0.19 and 0.94, respectively). Hyperpigmentation and grade 1+ acute toxicities were less frequent with UHFRT alone compared to UHFRT+ET (32% vs. 50%, 58% vs. 74%, p=0.04, 0.01, respectively). In multivariate analysis, grade 1+ acute toxicity was only significantly associated with younger age (OR=0.93, 95% CI 0.89-0.98, p=0.006), but not tumor size, ET compliance or RT regimen (p=0.76, 0.11 and 0.20, respectively).

Conclusion: UHFRT offers a convenient and easily tolerated treatment approach for older women who chose to omit or don’t comply with ET. Without stringent ET compliance, UHFRT alone, delivered via either WBI or APBI regimens, resulted in excellent tumor control, low toxicities, and good cosmetic results.