Breast cancer remains a leading malignancy among women globally. In addition to established factors like histopathology, hormone receptor status, and lymph node involvement, tools such as immunohistochemistry and molecular tests have been developed to assess tumor behavior and recurrence risk.

This study investigates the concordance between the Ki-67 proliferation index measured by immunohistochemistry and the recurrence risk scores obtained from molecular genomic testing in patients with invasive breast cancer.

This cross-sectional study included patients with invasive breast carcinoma at St. Luke’s Medical Center from 2019 to 2024, who underwent biopsy or mastectomy, with hormone status and Ki-67 index assessed by immunohistochemistry. All patients also had molecular genomic testing using either MammaPrint or OncotypeDX. Concordance between Ki-67 and the genomic recurrence risk score was evaluated using Kappa statistics, and results were further analyzed according to clinical risk and hormone receptor status.

Fifty-eight (58) patients met the study criteria. Most had grade 2, hormone receptor-positive, HER2-negative, and node-negative tumors, with high clinical risk based on Adjuvant! Online criteria (adapted from the MINDACT trial). The agreement between categorical Ki-67 and molecular recurrence risk was only fair: 66.7% for MammaPrint (kappa=0.35) and 60% for OncotypeDX (kappa = 0.29) using a 30% Ki-67 cutoff.

There is a fair agreement between Ki-67 and the molecular genomic tests. These findings are consistent with prior studies reporting weak to moderate association. Despite the limited sample size, Ki-67 remains a practical and accessible risk stratification tool, particularly where genomic assays are unavailable. The study supports integrating Ki-67 with clinicopathologic and genomic data to guide therapy, reflecting current best-practice recommendations.