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Life sciences · Journal article
Cancers · October 10, 2026
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Background: HER2 status is a dynamic process where one in ten patients with invasive breast cancer may undergo receptor conversion. We characterized HER2 status on circulating tumor cells (CTCs) in patients with HER2-low breast cancer. Methods: Patients with HER2-low expression, defined as immunohistochemistry (IHC) score of 1+ or 2+ with negative HER2 fluorescent in situ hybridization, were enrolled from 2019 to 2023. All patients were treatment-naïve to HER2-targeted therapies. Blood draws were collected serially q3–6 months. CTCs were enumerated using CellSearch™ and HER2 expression on CTCs were detected using an anti-HER2 fluorescent antibody. Presence of ≥1 CTCs was considered positive. Wilcoxon rank-sum, Fisher’s exact test, and Kaplan–Meier method were used for statistical analysis. Results: Among 39 HER2-low patients, median age was 49.0 years (IQR 40.5–58.5). Patients were predominantly Non-Hispanic White (26, 66.7%), with node-positive (29/38, 76.3%), ductal (27, 69.2%), stage II-III (24, 61.5%) disease. Over half (23, 59.0%) had inflammatory breast cancer (IBC). Rates of ER and PR positivity were 56.4% (22) and 51.3% (20), respectively. Majority of patients underwent surgery (23 mastectomy, seven segmental mastectomy) with a pCR rate of 17.2%. Up to 53.8% (21) of patients had HER2-positive CTCs during treatment course. Of these, 13 (61.9%) and eight (38.1%) were present before and after surgery, respectively. There were no differences in age, BMI, grade, histology, lymphovascular invasion, IBC status, and pCR rate by CTC-HER2 status. At a median follow up of 3.8 years (95% CI: 2.6–4.9), patients with HER2-positive CTCs demonstrated a trend of worse prognosis compared to those with HER2-negative CTCs (5-year overall survival 59.8% vs. 88.9%, p = 0.11). Conclusions: In this study cohort, a high proportion of HER2-low expressors demonstrated HER2-CTC positivity. These findings support the potential use of HER2 expression on CTCs as a biomarker to personalize therapies among patients with HER2-low breast cancer.