Omission of sentinel lymph node biopsy in early stage luminal breast cancer: impact on adjuvant CDK4/6 inhibitor recommendation and oncological outcomes

BACKGROUND: Sentinel lymph node biopsy (SLNB) omission is safe in many patients with small, clinically and imaging node-negative (cN0/iN0), hormone receptor (HR)-positive, human epidermal growth factor receptor 2 (HER2)-negative breast cancer (BC). However, recruitment of the pivotal SOUND/INSEMA trials ended prior to approval of adjuvant cyclin-dependent kinase 4/6 inhibitors (CDK4/6i). Therefore, the impact of SLNB omission on eligibility for CDK4/6i and associated oncological outcomes is unknown. METHODS: Single-center, retrospective study including patients with ≤cT2, cN0/iN0, HR-positive/HER2-negative early BC who underwent breast-conserving surgery and SLNB between 01/2014-12/2024. CDK4/6i eligibility was based on monarchE inclusion criteria and FDA approval (abemaciclib) and NATALEE inclusion criteria (ribociclib). Predictors of CDK4/6i eligibility and node-positivity were identified using multivariable logistic regression analysis. Impact on invasive disease-free survival was estimated based on published number needed to treat values in stage II disease (abemaciclib 28; ribociclib 63). RESULTS: Among 309 patients, eligibility was 6.1 % (19/309), 3.2 % (10/309), and 7.8 % (24/309) for abemaciclib (monarchE), abemaciclib (FDA approval), and ribociclib based on node-criteria; consequently, the number of SLNB necessary to identify one eligible patient was 17 (95 %CI 11-27), 31 (95 %CI 17-64) and 13 (95 %CI 9-20). Lymphovascular invasion was identified as a predictor for CDK4/6i eligibility and node-positivity. Omitting SLNB could increase recurrences by 0.3 % (95 %CI 0.2-0.4), 0.2 % (95 %CI 0.1-0.3), and 0.2 % (95 %CI 0.1-0.2) due to missed treatment with abemaciclib (monarchE), abemaciclib (FDA approval), and ribociclib, respectively. CONCLUSION: Omission of SLNB in patients with small HR-positive/HER2-negative tumors results in a missed indication for CDK4/6i in <8 % with minimal impact on recurrence.