Background: HER2 is the target of the therapeutic agents which are used to treat HER2-positive breast cancer. Reports have shown that the HER2 oncogene expression and its association with clinicopathological factors remain unclear in breast cancer (BC) patients. This study aimed to determine the correlation between HER2 expression and clinicalpathological characteristics of breast cancer in Vietnamese women. Methods: Between June 2016 and August 2018, paraffin-embedded specimens from 237 patients with primary invasive breast carcinoma in Hue University Hospital and Hue Center Hospital, Hue city, Vietnam were examined for pathological features. The gene expression of HER2, ER, PR and Ki-67 were determined by immunohistochemistry (IHC). The gene amplification of Her2 was assessed by using Dual color in situ hybridization (DISH). Results: The most frequent histological type was invasive carcinoma of no special type (NST) with 77.35%, the highest percentage of patients with Grade II was detected (59.36%), tumor size > 2 cm accounted for 71.31% of cases, Lymph node metastases were available in 57.86% cases. Most patients were diagnosed at stage II (59.18%). The majority of patients were classified as moderate Nottingham prognostic index (54.9%). Estrogen receptor and Progesterone receptor were positive in 53.16% and 50.63%, respectively. 76.37% of cases were in high expression group of Ki-67 (≥14%). HER2 IHC 2+, 3+ were accounted for 28.69% and HER2 gene amplification was detected in 31% cases. HER2 gene amplification and/or overexpression was significantly associated with cell proliferation index Ki67. Furthermore, HER2 gene expression tended to be more frequently found in tumors with large tumor size, high grade, high stage and high Nottingham prognostic index and confirmed their prognostic independent role. Conclusions: Our data indicated that HER2 gene expression was significantly correlated with cell proliferation index Ki67, but not significantly associated with another clinicopathological factors in breast cancer of Vietnamese women.

Breast cancer is the most common cancer among women, with an increasing incidence in most countries, representing a public health threat (Momenimovahed and Salehiniya, 2019;Zahmatkesh et al., 2016). In the United States, breast cancer caused 42,000 deaths in 2017 (Siegel et al., 2020). Clearly, there is a link with ageing, especially among women aged 45 to 65, and it is increasing among younger women (Bouchardy et al., 2007;Dobi et al., 2011;Zubor et al., 2006). Breast cancer under the age of 40 accounts accounts for 3-7% of all breast cancer cases