Abstract PO2-26-01: Prognostic Role of HER2 Expression in Patients with ER-positive/HER2-negative Breast Cancer. Results from a Population-Based Cancer Registry Study

Abstract Background: Estrogen Receptor (ER)-positive (+)/Human Epidermal Growth Factor Receptor 2 (HER2)-negative (-) breast cancers (BCs) express variable protein levels of HER2, which can influence prognosis. Materials and Methods: This cohort study was conducted using data on all consecutive patients (pts) diagnosed with BC between 2005 and 2017, which were systematically and prospectively collected by the Emilia-Romagna Cancer Registry (Provinces of Piacenza, Parma, Modena, and Ferrara), Italy. The study included 13527 pts with ER+/HER2- BC. Tumors were classified by HER2 IHC score (0 n=7155, 1+ n=5186, or 2+ with negative FISH n=1186). Comparisons of clinicopathologic characteristics and disease outcome were performed. Results: BCs with late-stage diagnosis, high histological grade, or high proliferative rate were more likely to be HER2 1+ or 2+/FISH- in comparison with earlier stage, low-grade, or low-proliferative tumors (lower bounds of 95% confidence intervals CIs for odds ratios ORs 1). BCs with high expression of ER (≥ 80% n=12383) were more enriched with HER2 1+ (OR 1.39; 95%CI, 1.2-1.6) or 2+/FISH- tumors (OR 1.26; 95%CI, 1.0-1.6) than ER-low/moderate (1-79% n=1144) ones. The 5-year overall survival (OS) for HER2 1+ BCs was lower than that for HER2 0 or 2+/FISH- tumors (P = 0.0018). This finding was confirmed also after stratification by ER status (low/moderate and high expression; P = 0.07 and 0.007, respectively). By multivariate logistic regression, the following variables were significantly associated with HER2 1+, or 2+/FISH- expression compared to HER2 0 tumors: age at diagnosis 50, high histological grade, ER expression ≥ 80% (lower bounds of 95% CIs for ORs 1). Multivariate Cox's regression analysis showed that histological grades G2 and G3 were independently associated with poor survival vs. G1 tumors (HR 1.2; 95%CI, 1.1-1.3 and 1.3; 95%CI, 1.1-1.4, respectively). Furthermore, HER2 2+/FISH- status was significantly associated with better survival in comparison with HER2 0 tumors (HR 0.82; 95%CI, 0.7-0.9). This finding was confirmed also after stratification by ER status (ER 1-79% and ≥ 80%). No significant difference in OS was observed between HER2 1+ and 0 BCs. Conclusions: The putative worse prognostic impact of HER2 expression in pts with ER-positive/HER2-negative BCs was not confirmed. The better outcome observed in pts with HER2 2+/FISH- BCs may be related to the known FISH-negative (HER2-non-amplified) status of this subgroup. These findings may help identify optimal patient inclusion criteria for clinical trials with novel anti-HER2 therapies in ER-positive/HER2-negative disease. Citation Format: Antonino Musolino, Olga Serra, Benedetta Pellegrino, Chiara Tommasi, Daniele Zanoni, Fabio Faccini, Maria Michiara, Paolo Sgargi, Giuliano Carrozzi, Stefano Ferretti, Veruska Grossi, Alessandra Ravaioli, Federica Zamagni, Fabio Falcini. Prognostic Role of HER2 Expression in Patients with ER-positive/HER2-negative Breast Cancer. Results from a Population-Based Cancer Registry 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 PO2-26-01.