Implementation of HER2DX Scores into Treatment Decisions in Early-Stage HER2-Positive Breast Cancer

HER2DX is a 27-gene genomic assay generating three complementary scores: a pCR score predicting the likelihood of achieving pathological complete response (pCR, defined as absence of residual invasive disease after neoadjuvant therapy), a Risk score estimating long-term recurrence risk, and an ERBB2 mRNA score quantifying HER2 transcriptional activation. Together, these scores may guide treatment escalation or de-escalation strategies in routine practice. We performed a single-center observational study at 12 de Octubre University Hospital (Madrid, Spain), including patients with early-stage HER2-positive breast cancer who underwent HER2DX testing (2023–2025), and analyzed clinicopathologic features, treatment decisions, and pathological outcomes. Forty-five patients were included (median age 57 years). Stage II accounted for 71.1% of cases; 66.7% were hormone receptor-positive, and 31.3% were node-positive. HER2DX modified clinical management in 51.1% of patients. The pCR score changed the initial strategy (neoadjuvant therapy versus upfront surgery) in 17.8% of cases and, independently, favored de-escalation to taxane plus dual HER2 blockade, with 90% of high-score tumors achieving a pathological complete response. The Risk score informed chemotherapy backbone selection within stage II disease. The ERBB2 score correlated with HER2 immunohistochemical intensity. In this exploratory real-world cohort, HER2DX provided biologically distinct information beyond traditional immunohistochemistry and was associated with modifications in multidisciplinary treatment decision-making in early-stage HER2-positive breast cancer, warranting prospective validation in larger cohorts.