Discordant pathologic response to neoadjuvant human epidermal growth factor receptor 2-directed therapy in primary breast tumour and axillary lymph node metastases.

Neoadjuvant chemotherapy combined with human epidermal growth factor receptor 2 (HER2)-directed therapy is associated with high rates of pathologic complete response in patients with HER2-positive breast cancer. Response in the primary tumour is often assumed to reflect systemic treatment efficacy. However, metastatic disease in axillary lymph nodes may respond differently.We report a case of HER2-positive breast cancer demonstrating marked histological treatment response in the breast but persistent viable metastatic carcinoma in multiple axillary lymph nodes without histological evidence of treatment effect. This discordant response highlights the potential for biological heterogeneity between primary and metastatic disease and underscores the importance of independent pathological evaluation of nodal disease after neoadjuvant treatment.Recognition of discordant response between the primary tumour and nodal metastases is important for accurate staging, prognostic assessment and multidisciplinary treatment planning.