Reliability of Ki67-Labeling Index in Core Needle Biopsy Specimens of ER+/HER2- Breast Cancers

The reliability of Ki67-labeling index (LI) in core needle biopsy (CNB) of ER+/HER2- invasive breast carcinoma (IBC) and factors affecting the discordance of Ki67-LI between CNB and surgical resection (SR) remain unsolved. We aimed to elucidate factors influencing the discordance of Ki67-LI between CNB and SR to classify ER+/HER2- IBC into luminal A-like (LumA) and luminal B-like (LumB). The cohort included 326 ER+/HER2- IBCs available with Ki67-LI data at both CNB and SR specimens. Survival analysis was performed on 122 patients. Spearman's rank correlation coefficient of Ki67-LI between them was 0.683. The log-rank test showed that patients with ER+/HER2- IBC with ≥ 20% Ki67-LI at CNB (p < 0.001) and SR specimens (p < 0.001) had significantly shorter disease-free survival. In multivariate analysis, a negative PgR (p = 0.002) and > 2 cm pathological tumor size (p < 0.001) had the most significant effect on the discordance of Ki67-LI between CNB and SR at 20% and 30% cutoffs, respectively. Histological grade III had a significant effect on the concordance between them (p = 0.01) at 20% cutoff. Ki67-LI assessment in CNB may be useful for classifying ER+/HER2- IBC into LumA and LumB with caution of some prognostic factors and cutoffs.