Deciphering HER2-low breast cancer (BC): insights from real-world data in early stage breast cancer.

Background: Human epidermal growth factor receptor 2 (HER2)-low has emerged as a potential new entity in breast cancer (BC). Data on this subset are limited, and prognostic results are controversial, evidencing the need of further data in a BC real-world cohort.

Methods: Patients with HER2-negative stage I-III BC diagnosed between 2006 and 2016 were retrospectively reviewed in a single cohort from the Catalan Institute of Oncology Badalona. Demographics and clinicopathological characteristics were examined via medical charts/electronic health records. We aim to describe and compare HER2-0/HER2-low populations through Chi-square or Fisher test, and explore its prognostic impact using Kaplan-Meier curves and Cox regression models.

Results: From a cohort of 1755 BC patients, 1401 invasive HER2-negative, stage I-III cases were evaluated. 87% were hormone receptor (HR)-positive versus 13% triple negative (TNBC). Overall, 43% were HER2-0 and 57% HER2-low (61% immunohistochemistry (IHC) 1+ and 39% IHC 2+). Comparing HER2-low versus HER2-0, HER2-low showed higher proportion of estrogen receptor (ER)-positive (91.6% vs 79.9%, p ⩽ 0.001) and progesterone receptor (PR)-positive (79.8% vs 68.9%, p ⩽ 0.001) cases. HER2-0 exhibited higher proportion of TNBC (20.1% vs 8.4%, p = 0.001), grade III tumors (28.8% vs 23.5%, p = 0.039), and higher Ki67 median value (26.47% vs 23.88%, p = 0.041). HER2-low was associated with longer time to distant recurrence (TTDR) compared to HER2-0 (67.8 vs 54.1 months; p = 0.015) and better BC-related survival (19.2 vs 16.3 years; p = 0.033). In the multivariable analysis, HER2-low was not an independent prognostic factor for TTDR and BC-related survival. ER expression showed a strong association with longer TTDR (Hazard Ratio: 0.425, p ⩽ 0.001) and improved BC-related survival (Hazard Ratio: 0.380, p ⩽ 0.001). PR expression was also associated with longer TTDR (Hazard Ratio: 0.496, p ⩽ 0.001), and improved BC-related survival (Hazard Ratio: 0.488, p ⩽ 0.001). Histological grade III was significantly associated with shorter TTDR (Hazard Ratio: 1.737, p = 0.002). Positive nodal status was the strongest factor correlated with worse BC-related survival (Hazard Ratio: 2.747, p ⩽ 0.001).

Conclusion: HER2-low was significantly associated with HR-positive disease, whereas HER2-0 group had higher incidence of TNBC, histological grade III and higher Ki67%. Although HER2-low group was associated with longer TTDR and improved BC-related survival, these findings could be explained by the greater proportion of favorable prognostic features in this subgroup compared to HER2-0.