LAUSR

Background: Based on data from randomized controlled trials, women age 65 or older with HR+, cN- breast cancer can omit post-lumpectomy RT if they take ET for 5 years. However ET non-compliance is a significant challenge, especially in non-trial patients. Initiation of and persistence with ET in the setting of RT omission is poorly understood in a non-trial patient population. Methods: In this retrospective cohort study from a single institution, women age 65 and older with HR+, cN- breast cancer who were diagnosed in 2005-2018 were included. Patients who underwent mastectomy were excluded. Persistence with ET was defined as taking ET for a minimum of 57 months, until disease recurrence, or until the patient was lost to follow up. Recurrence was defined as any local, regional, or distant recurrence, and excluded contralateral breast cancer. Multinomial logistic regression was used to examine trends in practice patterns over time and by age. Kaplan-Meier estimators were used to estimate the probability of ET discontinuation. Cox proportional hazards models were constructed to assess associations between recurrence and ET/RT. Results: Of 484 patients, median age was 71.9 years old and 90% were non-Hispanic white. The majority of patients had tumor size ≤2cm (81%) and did not receive chemotherapy (92%). Approximately one third were clinically node negative but did not undergo axillary surgery (32%). The average follow-up from date of diagnosis was 34.5 months. Percentages of patients who initiated ET and/or received RT are provided in Table 1. Receipt of both ET and RT decreased with age (p Conclusion: This real world data analysis showed that among older patients with cN- HR+ breast cancer, a substantial proportion of patients who omitted RT also did not initiate ET. Those who did not undergo RT were more likely to discontinue ET early. In addition, among women who omitted RT, those who did not initiate ET had higher risk of recurrence. Caution should be taken when omitting RT in this patient population, since concomitant omission of RT and poor compliance with ET could increase the risk of disease recurrence. Citation Format: Mei Wei, Xuechen Wang, Danielle Zimmerman, Lindsay M Burt, Benjamin Haaland, N. L. Henry. Endocrine therapy (ET) and radiotherapy (RT) use among elderly women with hormone receptor positive (HR+), clinically node negative (cN-) breast cancer [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P2-18-04.