Abstract P5-09-03: Real World Data on Neoadjuvant Endocrine Treatment and Ki67 Assessment in ER+/HER2- Breast Cancer. Results from a Single-Institution Prospective Cohort Study

Abstract

Purpose: Neoadjuvant endocrine treatment (NET) has become a useful tool for the downstaging of luminal-like breast cancers in postmenopausal patients. It enables us to increase breast conserving surgery (BCS) rates and provides an opportunity for assessing in vivo NET effectiveness and studying any biological changes that may act as valid biomarkers. The purpose of this study was to evaluate the effectiveness of NET as well as to assess the role of Ki67 proliferation rate changes as an indicator of endocrine responsiveness. Methods: From June 2016 to January 2022, a single-institution cohort of patients treated with NET and further surgery was evaluated. In patients with Ki67≥10%, a second core biopsy was performed after four weeks. Information regarding histopathological and clinical changes, as well as surgical management, was gathered. Results: A total of 168 estrogen receptor positive (ER+)/HER2 negative patients were included. The median age at diagnosis was 69.5 years old (IQR: 16.0). The median treatment duration was 5.0 months (IQR: 4.0). Median maximum size in the surgical sample was 44.0% smaller than pretreatment size measured by ultrasound (p<.0001), showing an inverse linear relationship with treatment duration. Median pretreatment Ki67 expression was 20.0% (IQR: 18.0) and was reduced to 5.0% (IQR: 8.0) after four weeks, and to 2.0% (IQR: 7.25) in the surgical sample (p< 0.0001). Other significant downgrading changes were observed with respect to tumor grade (p< 0.0001) and progesterone receptor (PR) expression (p< 0.0001). BCS was performed on 145 patients (86.3%). One case of pathological complete response was recorded. A larger Ki67 fold-change after four weeks was significantly related to a PEPI score of 0 (p< 0.002). No differences were observed between luminal A- and B-like tumors with regard to fold-change and PEPI score. No treatment abandonment was produced during the study. Conclusions: In our cohort, NET has proven effective for tumor size and Ki67 downstaging. This results in a higher rate of conservative surgery, aids in therapeutic decision-making, provides prognostic information, and constitutes a safe and well-tolerated approach Biological changes after NET Patient and tumor characteristics Citation Format: Covadonga Marti, Laura Frias, Adolfo Loayza, Elisa Moreno, Marcos Meléndez, Jose Ignacio Sanchez-Mendez. Real World Data on Neoadjuvant Endocrine Treatment and Ki67 Assessment in ER+/HER2- Breast Cancer. Results from a Single-Institution Prospective Cohort Study [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P5-09-03.