Abstract PO2-02-07: Clinicopathological Features and Prognostic Role of HER2 Low in Early Breast Cancer: A real world study

Abstract Background: The definition of human epidermal growth factor receptor-2(HER2) low was proposed with the development of T-DXd, a novel antibody-drug conjugate (ADCs) targeting HER2 in the field of breast cancer (BC) treatments. However, whether HER2 low is a distinct subgroup is still an ongoing debate with mixed results. Current retrospective studies focused on HER2 low concentrated more on metastatic BC, less was known in early BC. The retrospective study was conducted based on HER2 low early BC to comprehensively analyze its clinicopathological features and prognostic roles compared with HER2 IHC0. Methods: This single institution retrospective study enrolled 999 early stage (stage I through III) HER2 negative invasive BC patients diagnosed at Chinese PLA General Hospital from January 2010 to December 2015. Tumors with HER2 immunohistochemical (IHC) score of 1+ or 2+ with negative in situ hybridization assay were defined as HER2 low. Clinicopathological characteristics s and survival outcomes including disease-free survival (DFS) and overall survival (OS) were compared between HER2 IHC0 and HER2 low groups. The correlations between HER2 expression and estrogen receptor (ER) status were determined by Mantel-Haenszel χ2 test. Cohen’s kappa coefficient (k) was adopted to evaluate the concordance between HER2 expression on primary tumors and matched biopsies. Results: 999 patients were eligible for inclusion criteria, among which the percentages of HER2 low in the whole cohort, HR positive group and HR negative group were 83.6%, 86.54%, 72.14% respectively. It was observed the presence of a significant higher proportion of ER level 10% (p 0.001), PgR positive (p 0.001) and more patients who received endocrine therapy (p = 0.002). In HR positive group, clinicopathological features were balanced between HER2 IHC0 and HER2 low. In HR negative group, HER2 low manifested a higher proportion of invasive ductal type (p = 0.016) compared with HER2 IHC0. Survival analysis showed a significantly longer OS in HER2 low BC than HER2 IHC0. (HR: 0.458, 95%CI: 0.262-0.800, p = 0.0005). Nevertheless, the independent role was not reached in DFS. When stratified by HR status, patients with HER2 low in HR positive group demonstrated a longer OS than HER2 IHC0 (HR: 0.55, 95%CI: 0.273-1.105, p = 0.0385). The same trend was observed in HR negative group (HR: 0.385, 95%CI: 0.158-0.939, p = 0.0132). Neither the HR positive group nor the HR negative group achieved significant DFS. Besides, a positive correlation was observed between ER status and the rate of HER2 low (Mantel-Haenszel c2 test, p 0.001, Pearson’s R = 0.159, P 0.001). No regularity of survival differences between HER2 IHC0 and HER2 low was found in each ER level in the exploratory study. Finally, 119 patients who developed relapsed or metastatic disease were identified, with the most prevalent metastatic site being bone (31.1%), following by lung (16.80%) and lymph node (13.44%). Among 52 patients with matched biopsy samples, dynamic changes of HER2 IHC status were confirmed in both primary and relapsed statuses, with an discordance rate of 28.84% (K = 0.194, 95%CI: 0.168-0.219). conclusion: HER2 low expression breast cancer may not be regarded as a unique BC group in this real-world population due to similar clinicopathological features and prognostic roles especially in hormone-receptor positive cases. ER level was positively correlated with the rate of HER2 low population. Overall, whether HER2 low is a distinct subgroup remains to be supported and validated by more data both in clinical and molecular levels. Key words: HER2 low; Breast cancer; Clinicopathological Features; Prognosis; Molecular type Clinicopathological Features and Prognostic Role of HER2 Low in Early Breast Cancer: A real world study Citation Format: Ziqing Kong, Weihong Zhao. Clinicopathological Features and Prognostic Role of HER2 Low in Early Breast Cancer: A real world study abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO2-02-07.