Abstract PS3-09-04: The impact of axillary lymph node dissection on the indication of adjuvant abemaciclib in high-risk hormone receptor-positive breast cancer: Real-world analysis from a multicenter cohort in Brazil

Abstract Background: Adjuvant CDK4/6 inhibitor abemaciclib combined with endocrine therapy has demonstrated significant improvement in invasive disease-free survival among patients with high-risk, hormone receptor-positive, HER2-negative (HR+HER2-) early-stage breast cancer. Lymph node involvement is a key determinant of recurrence risk and plays a critical role in therapeutic decision-making. However, the optimal surgical management of axillary lymph nodes remains unclear when the extent of nodal involvement may influence adjuvant therapy decisions. This study aimed to investigate the proportion of real-world cases in which axillary dissection was essential for determining eligibility for adjuvant abemaciclib. Methods: We conducted a multicenter retrospective study of patients diagnosed with early-stage breast cancer between 2018 and 2025 within a national Brazilian cancer network. We evaluated the proportion of patients eligible for adjuvant abemaciclib based on phase 3 monarchE trial criteria. Indications were stratified into three sequentially applied groups (patients meeting a prior criterion were not reassigned to subsequent ones):1.Positive axillary lymph node(s) nodes and either grade 3 disease or tumor size ≥5 cm;2.Positive axillary lymph node(s) and Ki-67 ≥20%;3.Four or more positive axillary lymph nodes.Patients meeting only criterion 3 were considered reliant on axillary dissection for abemaciclib indication. Results: Among 656 patients with HR+/HER2- breast cancer, 284 (43.3%) met monarchE criteria for adjuvant abemaciclib. Median age was 49 years (range 24-83), with 52.8% being premenopausal. Ductal and lobular histologies were present in 72.2% and 16.5%, respectively. Tumor grade was 7.0% grade 1, 53.2% grade 2, and 34.5% grade 3. Clinical T stages were: 22.2% T1, 37.7% T2, 23.6% T3, and 6.7% T4. Clinical N stage was N0 in 23.2%, N1 in 49.3%, and N2-3 in 15.5%. Most patients (80.6%) had a Ki67-index ≥20%. Neoadjuvant chemotherapy was used in 46.8% of cases, and 51.1% received adjuvant chemotherapy. Axillary surgery comprised sentinel lymph node biopsy (51.4%) and axillary dissection (44.0%), with data missing in 4.6%. The distribution of criteria leading to abemaciclib indication is shown in Table 1. Discussion: This real-world study demonstrates that only a small subset of patients eligible for adjuvant abemaciclib met the criteria based solely on having four or more positive axillary lymph nodes. These findings support the notion that axillary dissection should not be pursued solely to establish eligibility for adjuvant CDK4/6 inhibitor therapy. Citation Format: L. Holland, L. Testa, C. Cavalcanti, J. F. Bessa, K. Cayres, P. H. Amor Divino, R. Naves, J. Bines, R. C. Bonadio. The impact of axillary lymph node dissection on the indication of adjuvant abemaciclib in high-risk hormone receptor-positive breast cancer: Real-world analysis from a multicenter cohort in Brazil abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS3-09-04.