Introduction:
This study aimed to assess the prognostic importance of HER2-low status in breast cancer (BC). Additionally, we compared the HER2-low subgroup (defined as HER2-low1+; HER2-low2+/ISH(-)) with HER2-zero and HER2-positive subgroups according to clinical, histopathological and molecular features.

Material and methods:
The study analyzed the medical records of BC patients treated at the National Institute of Oncology in Gliwice, Poland, between 2002-2018. HER2 overexpression was assessed in postoperative specimens or samples obtained via core needle biopsy.

Results:
Of the 657 tumors, 176 (27%) tumors were classified as HER2-low BC, 248 (38%) as HER2-zero and 233 (35%) as HER2-positive tumors. BC patients with HER2-low tumors were more likely to have hormone receptor-positive (HR+) status compared to those with HER2-zero tumors (82.4% vs. 62.1%, P < 0.001). Specifically, patients with HER2-low1+ tumors had a higher frequency of HR+ status versus both HER2-zero and HER2-positive tumors. Similarly, patients with HER2-low2+/ISH(-) tumors were more frequently characterized as HR+ compared to HER2-zero and HER2-positive patients. In the subgroup of patients with estrogen receptor negative tumors, OS was slightly worse than for those with HER2-low status compared to HER2-zero patients (5-year OS 72.6% vs. 86.7%, P = 0.074). A tendency toward poorer OS was noted in HER2-low1+ patients with negative prognostic factors, including HR-, progesterone receptor-negative, grade 3, and larger tumors.

Conclusions:
HER2-low status appears to be associated with worse OS in breast cancer patients with certain negative prognostic factors versus HER2-zero status. Also, HER2-low1+ and HER2-low2+/ISH(-) subgroups were characterized by HR+ status.