Abstract

595 Background: Neoadjuvant therapy significantly enhances the likelihood of breast-conserving surgery (BCS) in breast cancer patients. However, in ER-positive and HER-2 negative breast cancer, the response to neoadjuvant chemotherapy (NCT) is not as good as HER-2 positive or triple-negative breast cancer. In addition, with the evolution of multi-gene assays, many patients of this subtype may not require adjuvant chemotherapy. This study explores the potential of tailored neoadjuvant treatments (NCT or endocrine therapy (NET)) guided by 70-gene assay (Mammaprint, Agendia) to improve BCS rates in ER-positive/HER-2 negative breast cancer. Methods: A prospective, multicenter cohort study was conducted across four Korean centers from 2019 to 2023. Participants were clinical stage II-IIIA, ER-positive/HER2-negative breast cancer patients deemed unsuitable for BCS. Suitability was further verified by an independent three-surgeon panel using mammography or MRI. Out of 130 screened patients, 100 were enrolled. We analyzed 90 patients, excluding 10 cases; 2 with a basal type or HER2 type result in the blueprint, 7 who refused neoadjuvant therapy, and 1 who underwent surgery at another hospital. Before initiation of therapy, each surgeon recorded a target tumor size at which the surgeon would be able to do BCS considering location of the tumor and patient’s breast size. Based on the 70-gene assay results from core needle biopsies, high-genomic-risk patients received NCT (anthracycline + cyclophosphamide for 4 cycles, followed by 4 cycles of docetaxel), while low-risk patients underwent 16 weeks of NET with letrozole (±leuprolide for premenopausal women). Results: Out of the patients, 66 (73%) were high-genomic-risk and underwent NCT, while 24 (27%) received NET. Median tumor size was 3.7cm (85.6% clinical T2 and 14.4% T3/4). The primary endpoint, achieving the pre-established target tumor size for BCS, was reached by 70.0% (95% CI: 59.4-79.2%) of patients, significantly surpassing the set goal of 50.8% (a 15% increase from historical control). The rates were 74.2% for NCT and 58.3% for NET. The actual BCS rate was 58.9% overall (63.6% for NCT and 45.8% for NET). Clinically, 5.6% exhibited complete response (CR), 74.4% partial response (PR), 18.8% stable disease (SD), and 1.1% progressive disease (PD). Pathologic complete response (pCR) was achieved in 2.2% (all in the NCT group). No severe adverse events beyond expectations were observed. No pre-treatment clinical or pathological factor was significantly predictive for the BCS conversion. Conclusions: For ER positive, HER2 negative breast cancer patients initially ineligible for BCS, the use of pre-treatment multi-gene assays to guide selective NCT or NET could substantially increase BCS rates and reduce unnecessary chemotherapy. Clinical trial information: NCT03900637 .