LAUSR

National Comprehensive Center Network guidelines recommend radiotherapy (RT) omission in women age ≥ 70 years with estrogen receptor-positive (ER+), cN0, T1 tumors post-lumpectomy if they receive endocrine therapy (ET). However, little is known about the impact of poor adherence on locoregional recurrence (LRR) in elderly women forgoing RT. Women age ≥ 70 years with pT1–2 ER+ breast cancer undergoing lumpectomy without RT from 2004 to 2019 were identified from a prospectively maintained database. ET adherence, calculated as treatment duration over follow-up time up to 5 years, was determined by chart review. We compared clinicopathologic characteristics and rates of LRR between women with high adherence (≥ 80%), low adherence (< 80%), and no ET. Of 968 women (27 bilateral cancers), adherence was high in 676 (70%) and low in 162 (17%); 130 (13%) took no ET. Younger age and use of aromatase inhibitor were associated with high adherence. On multivariable analysis, tumor size (hazard ratio [HR] 1.67, 95% confidence interval [CI] 1.03–2.68, p = 0.04) and high adherence (HR 0.13, 95% CI 0.07–0.26, p < 0.001) were significantly associated with LRR. At 53 months median follow-up, the 5-year rate of LRR was 3.1% (95% CI 2.4–3.9%) with high adherence, 14.7% (95% CI 11.7–17.7%) with low adherence, and 17.9% (95% CI 13.9–21.8%) with no ET (p < 0.01). Although adherence to ET was high overall, in the 30% of women with low adherence or no ET, LRR rates were significantly increased. Counseling regarding the distinct toxicities of ET and RT can help patients choose the therapy to which they will likely adhere to.