Breast Cancer with HER2 IHC 2+ and Average HER2 Signals/Cell ≥ 6 and HER2/CEP17 Ratio < 2: A Multi-Institutional Cohort Analysis Emphasizing Outcome and Molecular Subtype

Breast cancer (BC) with average HER2 signals/cell ≥6 and HER2/CEP17 ratio <2 [in-situ hybridization group 3 (ISH group 3)] is very rare, accounting for 0.4-3.0% of cases sent for the dual-probe ISH assay. While such patients are currently eligible for treatment with HER2-targeted therapy, their characteristics and outcomes remain poorly understood. 62 BCs with equivocal HER2 immunohistochemical (IHC) score (2+) and reflex ISH group 3 results were identified across four institutions. Available clinicopathological characteristics, MammaPrint and BluePrint molecular results, and follow-up information were retrospectively analyzed. Most BCs with HER2 equivocal IHC and ISH group 3 results were histologic grade 2 or 3 (100%), estrogen receptor (ER) positive (90.3%), with an average HER2 signals/cell of 7.3. Molecular profiles revealed 80% (16/20) of tumors were luminal subtypes and HER2 molecular subtype was identified in 10% of tumors (2/20). Twelve patients (19.4%) developed local recurrence and/or distant metastasis with a median follow-up of 50 months. One of 10 patients (10%) achieved pathologic complete response (pCR) after neoadjuvant chemotherapy. Forty-nine (79%) patients completed anti-HER2 agents, and exploratory analysis showed no statistically significant difference in disease outcomes between patients who completed anti-HER2 treatment and those who did not. Univariate analysis revealed advanced clinical stage and ER/PR negativity were associated with unfavorable disease outcomes, and exploratory multivariate analysis demonstrated clinical stage was the most significant factor associated with disease outcomes in the studied population. These findings increase our understanding of this rare, but clinically important HER2 category. Large-scale prospective randomized studies are needed to further evaluate the role of perioperative HER2-targeted therapy in this patient population.