Objectives: Breast carcinomas overexpressing human epidermal growth factor receptor 2 (HER2) are typically associated with higher tumor grade and faster progression. HER2 positivity is rare in low-grade breast carcinomas with unclear biological implications. We aimed to characterize their clinicopathologic and molecular profiles in this study.
Methods: There were 2 cohorts of Nottingham grade 1, HER2-positive invasive breast carcinomas examined: (1) an institutional series (n = 14) and (2) tumors from patients (n = 59) enrolled in the FLEX multicenter clinical registry with MammaPrint and BluePrint profiling.
Results: Most (79%) in the case series were both estrogen receptor (ER) and progesterone receptor (PR)-positive. Over half were pathologic or clinical T1N0 tumors. In the 9 cases with adequate material for next-generation sequencing, the majority (66%) demonstrated ERBB2 copy number variations. Most (66%) received HER2-targeted therapy. No recurrences were observed, with a median follow-up time of 43 months. In the FLEX cohort, most tumors were ER-positive (86%) and PR-positive (68%), and over half were clinical T1. Most (70%) were of the luminal phenotype, and over half (54%) were low-risk on MammaPrint.
Conclusions: Low-grade HER2-positive breast carcinomas constitute mostly low-stage, luminal-type, and apparently low-risk tumors, warranting investigation into whether therapy de-escalation could achieve favorable outcomes with less toxicity in this population.
Keywords: HER2-directed therapy; HER2-positive; invasive breast carcinoma; low-grade and early-stage breast carcinoma.
© The Author(s) 2025. Published by Oxford University Press on behalf of American Society for Clinical Pathology. All rights reserved. For commercial re-use, please contact reprints@oup.com for reprints and translation rights for reprints. All other permissions can be obtained through our RightsLink service via the Permissions link on the article page on our site—for further information please contact journals.permissions@oup.com.