Associations of the HER2DX Genomic Test with Biological and Pathological Features in HER2-positive Breast Cancer.

BACKGROUND: HER2DX is a validated genomic assay used to support treatment decisions in early-stage HER2-positive (HER2+) breast cancer. It provides three scores: relapse risk, likelihood of pathologic complete response (pCR), and ERBB2 mRNA expression. This study aimed to evaluate the association between HER2DX and histopathologic features and to assess its relationship with pCR after neoadjuvant therapy. METHODS: Patients with newly diagnosed stage I-III HER2+ breast cancer were analyzed based on available HER2DX results during routine care in Spain (January 2022-June 2025). Centralized HER2DX testing was performed on formalin-fixed, paraffin-embedded tumor samples. Histopathologic analysis included tumor grade, hormone receptor (HR) status, histological subtype, Ki67 index, HER2 immunohistochemistry (IHC) score, stromal tumor-infiltrating lymphocytes (TILs), tertiary lymphoid structures (TLS), and spatial immune distribution. Univariate and multivariable logistic regression analyses were conducted to identify factors associated with pCR after neoadjuvant trastuzumab-based therapy. RESULTS: A total of 410 HER2+ tumors were analyzed, and 250 patients received neoadjuvant trastuzumab-based therapy with available surgical outcomes (36.0% achieved a pCR). HER2DX pCR scores were significantly associated with all eight histopathologic features, whereas relapse risk and ERBB2 scores were associated with five and two, respectively. TILs correlated with the immune/immunoglobulin (IGG) signature (r=0.59), and Ki67 with the proliferation signature (r=0.50). The HER2DX pCR score remained the only independent predictor of pCR in multivariable analysis (odds ratio [OR]=1.77, 95% CI 1.08-2.97, p=0.030). CONCLUSIONS: HER2DX reflects key biological and pathological features of HER2+ breast cancer and independently predicts pCR, supporting its utility for individualized treatment decision-making.