Abstract PD12-03: Omission of sentinel lymph node biopsy in early stage HR-positive HER2-negative breast cancer: impact on adjuvant CDK4/6 inhibitor candidacy and oncological outcomes

Abstract Introduction The SOUND and INSEMA trials demonstrated that sentinel lymph node biopsy (SLNB) can be safely omitted in patients with small, hormone receptor (HR)-positive, human epidermal growth factor receptor 2 (HER2)-negative breast cancer (BC) who are clinically and imaging node-negative (cN0/iN0). However, recruitment for these trials ended prior to the approval of adjuvant cyclin-dependent kinase 4/6 (CDK4/6) inhibitors, which rely in part on nodal status for eligibility. Therefore, the extent to which SLNB omission impacts eligibility for CDK4/6 inhibitors and consequently oncological outcomes is unknown. Methods This was a single-center, retrospective cohort study of patients with clinically T1 and T2, cN0/iN0, HR-positive/HER2-negative BC who underwent breast-conserving surgery with a SLNB between 01/2014-12/2024. Preoperative axillary ultrasound was used throughout the study period. CDK4/6 inhibitor eligibility was estimated based on the monarchE and NATALEE inclusion criteria. Abemaciclib eligibility included patients with ≥4 positive lymph nodes, or 1-3 positive lymph nodes and either grade 3 disease, tumor size ≥5cm, or Ki67 ≥20%. Ribociclib eligibility based on node-criteria was defined as either pT1pN1 (excluding pN1mi) or pT2pN1, provided that no additional criteria would render the pT2 status per se eligible for ribociclib treatment. Predictors of CDK4/6 inhibitor eligibility and node-positivity were identified utilizing multivariable logistic regression models. Estimates of the impact on invasive disease-free survival are based on the five- and three-year results of the monarchE and NATALEE trials, respectively. Results Of 309 patients, 78.3% (242/309) were postmenopausal, 28.8% (89/309) had a ≥pT2 tumor, and 14.6% (45/309) had pN1 disease. 21.4% (66/309) qualified for adjuvant CDK4/6 inhibitor treatment. 6.1% (19/309) were eligible for abemaciclib and 7.8% (24/309) were eligible for ribociclib based on node criteria. Consequently, 17 (95%CI 11-27) and 13 (95%CI 9-20) patients would be required to undergo SLNB in order to identify one patient qualifying for abemaciclib and ribociclib (based on node-criteria), respectively. Predictors of CDK4/6 inhibitor eligibility were found to be pathological tumor stage ≥pT2 (odds ratio [OR] 10.3, 95%CI 4.5-25.1, p