Abstract
IMPORTANCE: ERBB2-low (ie, ERBB2 immunohistochemistry score of 1+ or 2+ in the absence of ERBB2 gene amplification) breast cancer (BC) is a new entity, with emerging dedicated treatments. Little is known about its prognosis and response to conventional therapy compared with ERBB2-zero breast tumors (ie, those with an immunohistochemistry score of 0). OBJECTIVE: To compare the outcomes for patients with ERBB2-low metastatic BC (MBC) with those of patients with ERBB2-zero MBC. DESIGN, SETTING, AND PARTICIPANTS: This cohort study was conducted from the Epidemiological Strategy and Medical Economics MBC platform and included patients with MBC treated between 2008 and 2016 in 18 French comprehensive cancer centers. The data analysis was conducted from July 16, 2020, to April 1, 2022. MAIN OUTCOMES AND MEASURES: The main outcome was overall survival (OS), and the secondary outcome was progression-free survival under first-line treatments (PFS1). RESULTS: The median (range) age was 60.0 (22.0-103.0) years. Among 15 054 patients with MBC, 4671 (31%) had ERBB2-low MBC and 10 383 (69%) had ERBB2-zero MBC. The proportion of ERBB2-low cancers was higher among patients with hormone receptor-positive MBC than those with hormone receptor-negative disease (4083 patients [33.0%] vs 588 patients [21.0%]). With a median follow-up of 49.5 months (95% CI, 48.6-50.4 months), the median OS of the ERBB2-low group was 38.0 months (95% CI, 36.4-40.5 months) compared with 33.9 months (95% CI, 32.9-34.9 months) for the ERBB2-zero group (P < .001). After adjustment for age, visceral metastases, number of metastatic sites, de novo disease, period of care, and hormone receptor status, patients with ERBB2-low MBC had slightly better OS compared with patients with ERBB2-zero MBC (adjusted hazard ratio, 0.95; 95% CI, 0.91-0.99; P = .02). In contrast, PFS1 did not differ by ERBB2 status (adjusted hazard ratio, 0.99; 95% CI, 0.95-1.02; P = .45). No significant differences in OS and PFS1 were observed in multivariate analyses by hormone receptor status and types of frontline treatment. CONCLUSIONS AND RELEVANCE: In this large cohort study, patients with ERBB2-low MBC had a slightly better OS than those with completely ERBB2-zero tumors, but identical PFS1, which could help guide treatment selection.