Clinical and pathological characteristics of HR+ breast cancer with HER2 low and HER2(0) expression: exploring endocrine therapy sensitivity.

Background: The clinical significance of human epidermal growth factor receptor 2 (HER2) low and HER2(0) expression in hormone receptor-positive (HR+) breast cancer patients remains uncertain. This study aimed to explore the clinical and pathological characteristics, prognosis, and endocrine therapy (ET) sensitivity among HR+ breast cancer patients with HER2 low and HER2(0) expression.

Methods: We conducted a retrospective analysis of 390 HR+, HER2-negative breast cancer patients who underwent radical surgery at The First Affiliated Hospital of Bengbu Medical University between December 2014 and December 2017. HER2 status was classified per ASCO/CAP 2018 guidelines: tumors with an immunohistochemistry (IHC) score of 0 were defined as HER2(0), and those with a score of 1+ or 2+ with negative FISH for ERBB2 gene amplification were categorized as HER2 low. Clinical and pathological characteristics, ET sensitivity, and survival outcomes were analyzed. Primary endpoints included disease-free survival (DFS) and overall survival (OS), with ROC curve analysis employed to determine prognostic thresholds.

Results: Of the 390 HR+ breast cancer patients, 32.6% had HER2(0) expression [HER2(0) cohort] and 67.4% had HER2 low expression (HER2-low cohort). Most baseline characteristics were balanced between the two cohorts, and the HER2(0) cohort had significantly worse DFS and OS than the HER2-low cohort (both P<0.05). Within the same pN stage (pN0-2), patients with HER2 low expression had a better prognosis (all P<0.05). But at pN3, patients had worse survival outcomes (P=0.003). Patients with the IHC score >4 had significantly better OS (P<0.001) and DFS (P=0.003). No significant difference in OS and DFS was observed in the IHC score ≤4 group between cohorts (both P>0.05), whereas in the IHC score >4 group, the HER2-low cohort had better OS (P<0.001) and DFS (P=0.01).

Conclusions: Patients with HER2 low expression have distinct clinical characteristics and a better prognosis compared to those with HER2(0) expression. Our study provides further real-world evidence for personalized treatment of breast cancer patients.