Clinical characteristics and prognostic impact of HER2 low expression in breast cancer subtypes from a Brazilian real-world cohort

Breast cancer (BC) is the most prevalent cancer among Brazilian women, yet related data remain limited. The HER2-low classification has gained significance with the advent of targeted therapies. This study aimed to assess survival outcomes of HER2-low BC compared to other subtypes in a real-world Brazilian cohort. We analyzed data from 8,485 breast cancer patients treated at Pérola Byington Hospital between 2010 and 2019. Overall survival (OS) was the primary endpoint, stratified by cancer subtype. The t-test and chi-square test evaluated variable associations, while multivariate analysis calculated odds ratios and 95% confidence intervals. Cox regression assessed survival, and Kaplan-Meier curves illustrated OS differences. The patients with HER2-low breast cancers showed significantly better overall survival than those with the triple-negative subtype (p < 0.01). However, they had significantly poorer overall survival than those with the Luminal A-like subtype (p < 0.01). The patients with triple-negative HER2-low disease had a higher risk of mortality than those with HER2-0 (p < 0.01). Finally, patients who achieved pathological complete response experienced significantly better overall survival than those who did not (p < 0.01). Our findings highlight triple negative HER2-low BC as a distinct subtype identifiable via standard immunohistochemistry, beyond just biomarker status. The study underscores the prognostic diversity among BC subtypes and emphasizes the importance of personalized treatment strategies.