LAUSR

Background: neoadjuvant endocrine treatment (NET) has demonstrated to be a useful strategy for downstaging luminal breast cancer as well as to deepen into luminal tumors biology. But, in spite of evidence, NET remains as an under-utilized tool, relegated to elderly frail patients. Purpose: to determine clinical and biological response after NET in luminal breast cancer.Methods: An observational prospective study was performed. Postmenopausal patients with luminal breast cancer T1-3, N0-2 diagnosed between January 2016 and April 2019 were included. They received treatment with letrozole 2,5mg/24h usually for 4-6 months or until maximum response was achieved. A few patients continued treatment further than 6 months because of surgery refusal or contraindication. In the rest of cases, surgery was performed after NET. An intermediate core biopsy (CB) was carried out in those patients with initial Ki67 >10%. Immunohistochemical and conventional pathological studies were made in the initial CB as well as in the intermediate CB and the surgical specimen.PEPI score (PS) was also calculated. Results: a total of 106 patients were included, with an average age of 73.7y [53-90]. 80 (75.5%) were ductal infiltrating carcinomas; 16(15,1%) lobular infiltrating carcinomas. The 10 remaining cases were minority histologic types. 62.3% were luminal B tumors. A positive axilla was present in 16 patients (15.0%). Surgery has been performed in 72 patients. The remaining 33 patients continue treatment nowadays (in 6 of them surgery was dismissed or refused by patient). Breast conserving surgery was feasible in 60 out of 72 (83.3%), although two of these patients chose mastectomy. Average size before treatment was 28.2mm [7.0-90.0], and in the surgical sample (pT) was 15.9mm [4.0-35.0] (p Citation Format: Covadonga Marti, Laura Frias, Adolfo Loayza, Laura Yebenes, Marcos Melendez, Elisa Moreno, Jose M Oliver, Pilar Zamora, Jose I Sanchez-Mendez. Biological and clinical changes after neoadjuvant endocrine treatment: Results in a Spanish cohort of 106 patients [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 P5-13-06.