Abstract P2-18-01: Single modality adjuvant therapy with radiation (RT) alone or endocrine therapy (ET) alone in women ≥65 years with early stage, hormone receptor positive breast cancer treated with breast-conserving surgery: A population-based analysis

Abstract Purpose: Current guidelines recommend for women (≥65 years at diagnosis) with early stage (T1N0), hormone receptor (HR+) breast cancer (BC) treated with breast-conserving surgery (BCS) that treatment with 5 years of endocrine therapy (ET) alone with omission of radiation (RT) is acceptable. Population studies however report that many women continue to receive RT. The use of RT may reflect preferences due to the convenience of modern RT and/or concerns of the adverse effects of ET. Adjuvant treatment with RT alone may be an attractive option, but further data on the outcomes and morbidity following mono adjuvant therapy with either RT alone or ET alone is needed. We established a contemporary population-based cohort of women ≥65 years with HR+ T1N0 BC treated by BCS. We report the risk of local recurrence, any secondary breast events and adverse treatment-related effects following single modality adjuvant therapy with RT or ET alone. Methods: We identified all women aged ≥65 years at diagnosis with HR+ T1N0 BC diagnosed in Ontario from 2010-2016, treated with BCS and adjuvant RT alone or ET alone (tamoxifen or aromatase inhibitors (AI)). The administration of ET was ascertained from the Ontario Drug Benefit database. Cases were coded as having received either tamoxifen or AI according to the first prescription. Cases with Her2+ BC or those who did not undergo nodal surgery were excluded. Outcomes were ascertained using deterministic linkages of administrative databases. Propensity-weighted scores (PS) were calculated using baseline factors (diagnosis year, age, Charlson comorbidity score, nodal surgery type, grade, socioeconomic status, HR category). Kaplan Meier curves and PS-adjusted multivariable Cox regression were used to evaluate risks of ipsilateral LR, any in-breast event (ipsilateral or contralateral), and adverse treatment-related outcomes (venous thromboembolism (VTE), osteoporotic fracture, vaginal bleeding requiring hysteroscopy or curettage, ischemic heart events, second malignancy. Results: The cohort includes 1054 that received BCS + adjuvant RT and 497 that received BCS + adjuvant ET alone (N= 201 tamoxifen; N=296 AI). Median follow-up time was 5 years. LR occurred in 0.9% (n=10) after RT alone and 2.8% (n=14) after ET alone (p=0.005). Rates of any in-breast event were 3.2% and 3.4%, respectively (p=0.8). Adjusting for PS, treatment with ET alone was associated with an increased risk of LR (HR= 2.79, 95% CI: 1.20, 6.49, p=0.02), no difference in the risk of any in-breast event (HR=0.96, 95% CI: 0.53, 1.77, p=0.9) and a higher risk of VTE (HR=1.8, 95% CI: 1.0, 3.2, p=.05) compared to those treated with RT alone. Women treated with tamoxifen alone had a 5-fold increased risk of vaginal bleeding (HR=4.7, 95%CI: 2.2, 10.0, p<0.0001) and a 2-fold increased risk of VTE (HR=1.9, 95% CI: 0.97, 3.75, p=0.06) compared to those treated with AI alone or RT alone. There were no significant differences in the risks of any or osteoporotic fracture, ischemic heart events, or second malignancies across treatment groups (Table 1). Conclusions: Women ≥65 years with HR+ EBC treated with single modality adjuvant therapy with RT or ET alone experience low risks of recurrence and morbidity. In this population-based study, RT alone compared to ET alone was associated with a lower risk of LR and less adverse events. Table 1. 5-year rates of adverse eventp-valueAny Fracture0.3• RT only11.9• tamoxifen9.5• aromatase inhibitor13.3Osteoporotic Fracture0.1• RT only5.8• tamoxifen2.8• aromatase inhibitor7.3Hysteroscopy/Curettage<.0001• RT only2.1• tamoxifen7.9• aromatase inhibitor0.5Thrombosis/Pulmonary Embolism0.1• RT only3.2• tamoxifen6.2• aromatase inhibitor3.8Ischemic Heart Disease0.3• RT only4.5• tamoxifen1.4• aromatase inhibitor4.2Other Cancer0.7• RT only6.7• tamoxifen7.7• aromatase inhibitor6.4 Citation Format: Mira Goldberg, Rinku Sutradhar, Lawrence Paszat, Timothy Whelan, Sumei Gu, Cindy Fong, Eileen Rakovitch. Single modality adjuvant therapy with radiation (RT) alone or endocrine therapy (ET) alone in women ≥65 years with early stage, hormone receptor positive breast cancer treated with breast-conserving surgery: A population-based analysis [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 P2-18-01.

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