Receptor Status Changes, HER2-Low Discordance, and TNBC Transformation in Metastatic Breast Cancer: Prognostic Significance and Impact on Survival Outcomes

Background/Objectives: Therapeutic options for metastatic breast cancer vary according to hormone receptor (HR) status, human epidermal growth factor receptor 2 (HER2) status, and, importantly, the newly recognized HER2-low status. In triple-negative breast cancer (TNBC), characterized by the absence of HR and HER2 receptors, therapeutic approaches and survival rates differ significantly. Current treatments have improved survival rates in metastatic diseases, resulting in an increased demand for secondary biopsies. The findings from these biopsies affect the range of therapeutic options. Methods: The data of 172 patients with metastatic breast cancer were retrospectively analyzed. The initial and subsequent biopsy specimens from these patients were compared. Their effects on survival and prognosis were analyzed statistically. Results: Overall, 12.8% of patients transitioned to triple-negative status. Upon analysis of HER2-low status, 13.4% of patients initially classified as HER2-negative transitioned to HER2-low status. The median overall survival was 26 months (95% CI: 19.1–32.8) in the HR-concordant group and 21 months (95% CI: 14.3–27.6) in the HR-discordant group (p < 0.03). Conclusions: The present study clarifies the rates of HR and HER2 receptor concordance and discordance in metastatic breast cancer. It provides a comprehensive dataset that incorporates all receptors and TNBC transformation. Furthermore, it demonstrates conversion to HER2-low status and examines the implications of TNBC transformation and HR discordance on prognosis and survival.