DOI: 10.1200/jco.2024.42.16_suppl.e13148 ISSN: 0732-183X

Landscape of HER2-low breast cancer: Insights from a six-year study on incidence and clinicopathological characteristics.

Michel Abou Khalil, Lea Habibian, Christine Martin, Karl Semaan, Abir Khaddage, Nadine EL Kassis, Carole Kesrouani, Hampig Raphael Kourie, David M. Atallah

e13148

Background: Human epidermal growth factor receptor 2 (HER2)-low breast cancer has emerged as a subtype of breast cancer, defined by HER2 1+ or 2+ in immunohistochemistry (IHC) and the absence of the ERBB2 gene amplification on fluorescence in situ hybridization (FISH). Recent trials showed marked response of HER2 breast cancer to novel anti-HER2 antibody-drug conjugates, rising interest in the HER2-low subtype. To date, data on characteristics of HER2-low breast cancer subtype is limited. Herein, we describe the clinicopathological characteristics of HER2-low breast cancer subtype compared HER2-zero/negative breast cancer. Methods: Real-world data from the Anatomic Pathology Division of Hotel Dieu de France, spanning 2017-2022, was retrospectively collected. Patients with HER2-IHC 3+ and HER2-IHC 2+ with ISH-amplified (HER2-positive) were excluded to compare HER2-low to HER2-zero breast cancer subtypes. Patients were then categorized based on their hormone receptor (HR) status. Clinicopathological characteristics between the groups were compared using a Chi-Square and Mann–Whitney U tests. Results: Out of 1195 patients, we observed 341 (28.5%) HER2-low breast cancers cases. HER2-positive breast cancer cases (n=178; 14.9%) were subsequently excluded. There was no significant difference in age and sex between the HER2-low and the HER2-zero group (p=0.3 and 0.8, respectively). HER2-low breast cancer was associated with positive estrogen receptor (ER) status and positive progesterone receptor (PR) status (p<0.001 and p=0.01, respectively). Ductal adenocarcinomas were more frequently observed in the HER2-low group, whereas lobular and metaplastic adenocarcinomas were especially prevalent in the HER2-zero group (all p<0.001). When stratified by HR status, 87.4% of patients had HR-positive status and 12.6% were HR-negative. Furthermore, the occurrence rate of tumors with a low Ki-67 labeling index was comparable between the two groups (p=0.9). However, among the HR-negative group, HER2-low tumors tended to show lower proliferation index compared to HER2-zero tumors (p = 0.04; Table). Conclusions: In conclusion, this study showed that HER2-low is a distinct from HER2-zero and is frequently present among patients with breast cancer. Clinicopathological features such as histological type differ between HER2-zero and HER2-low breast cancer. Our findings suggest that, within HR-negative breast cancer, breast cancer with low HER2 expression exhibits a less aggressive profile when compared to HER2-zero tumors. [Table: see text]