Concordance of Ki-67 index and histologic grade between biopsy and surgical specimens in breast cancer: Implications of treatment decision-making.

Abstract

e12593 Background: The Ki-67 index and histologic grade are key pathological features guiding systemic treatment decisions in Breast Cancer (BC). Variability between core biopsy and surgical specimens may impact management, including decisions regarding omission of sentinel lymph node biopsy. We evaluated the concordance of Ki-67 and histologic grade between biopsy and surgical specimens and quantified clinically meaningful Ki-67 differences. Methods: We conducted a retrospective analysis of BC patients with paired biopsy and surgical pathology specimens treated between 2023 and 2025 in a single oncology network in Brazil. Patients receiving neoadjuvant therapy were excluded. Ki-67 concordance was assessed using a 20% cutoff ( < 20% vs ≥20%) and by absolute differences > 10% between specimens. Concordance was evaluated using cross-tabulations and Fisher's exact tests. Surgical specimen grade was used as the reference for grade-specific Ki-67 analyses. Results: A total of 275 paired specimens were analyzed. Ki-67 ≥ 20% was observed in 49.1% of biopsy specimens and 54.2% of surgical specimens. In biopsy samples, 33.0% were grade 1, 53.3% grade 2, and 13.7% grade 3, compared with 23.6%, 58.2% and 18.2%, respectively, in surgical specimens. Median Ki-67 in biopsy samples was 12% (IQR 6-19%) for grade 1, 19% (IQR 10-30%) for grade 2, and 60% (IQR 30-70%) for grade 3. In surgical specimens, median Ki-67 was 12% (IQR 8-15%), 20% (IQR 13-29%), and 50% (IQR 25-60%), respectively. Overall, 25.1% of patients showed a Ki-67 difference > 10% bteween biopsy and surgery, including 10.9% with decrease and 14.2% with an increase in the surgical specimen. Ki-67 variation was most frequent in grade 3 tumors (20% decrease > 10%; 16% increase > 10%), but was also observed in grade 2 (8.8% decrease; 16.9% increase) and grade 1 tumors (9.2% decrease; 6.2% increase). Significant discordance was also observed for histologic grade (p < 0.001). Conclusions: Substantial variability in Ki-67 index and histologic grade exists between biopsy and surgical specimens, Approximately one third os cases crossed the 20% Ki-67 thershold, and histologic grade was upgraded to grade 3 in 13%. These findings highlight the limitations of relying solely on biopsy pathology for systemic treatment decisions, particularly in hormone receptor-positive disease, where Ki-67 and grade may influence adjuvant treatment considerations, including eligibility for CDK4/6 inhibitors and decisions regarding omission of sentinel lymph node biopsy. Routine reassessment of postoperative pathology may optimize treatment decision-making Surgery Ki-67 < 20% Surgery Ki-67 ≥ 20% p value Biopsy Ki-67 < 20% 98 (70.0%) 42 (30.0%) < 0.001 Biopsy Ki-67 ≥ 20% 28 (20.7%) 107 (79.3%) Surgery G1 Surgery G2 Surgery G3 Biopsy G1 53 (59.6%) 35 (39.3%) 1 (1.1%) < 0.001 Biopsy G2 10 (6.9%) 117 (81.3%) 17 (11.8%) Biopsy G3 1 (2.7%) 5 (13.5%) 31 (83.8%)