HER2-Ultralow: Prevalence, Characteristics, and Treatment Choices Among Advanced Breast Cancer Patients With Tumors Initially Scored as IHC 0.

Authors

Anushree R Iyengar, Erinn P Downs, Sandhya Mehta, Nivedita Rangarajan, Michele Sue-Ann Woo, Simone T Sredni, Rosemarie Di Donato, Safak Simsek, Darren M Wilson, Katherine Krieser, Elise Bieri Patzke, Natalie Kyek, Christian Anderson, Tyler Wagner, Jason Hipp, Aziza Nassar, Hannah Barman

Published in

The breast journal. Volume 2026. Issue 1. Pages e7212120.

Abstract

Historically, HER2 status in invasive breast cancer has been categorized as HER2-positive (IHC 3+, IHC 2+/ISH+) or HER2-negative (IHC 0, IHC 1+, IHC 2+/ISH-). Patients meeting IHC 0 with membrane staining (HER2-ultralow) criteria may benefit from HER2-targeted therapies such as trastuzumab deruxtecan. This cohort study assessed the prevalence of HER2-ultralow expression by re-scoring HER2 IHC slides using Mayo Clinic electronic health record data. Three hundred patients with advanced breast cancer (Stages III-IV) and documented HER2 IHC 0 status (January 2017-March 2023) were identified. One slide per patient was digitized and independently re-scored by two Mayo Clinic pathologists following the 2023 ASCO-CAP guidelines, including tumor staining percentage to denote HER2-ultralow status. A sensitivity analysis was performed by a third independent pathologist. The re-scored patients had a mean age of 57.7 years (SD = 13.6). Most samples (95%, n = 285) remained scored IHC 0 by at least one pathologist; 60% of these met HER2-ultralow criteria per at least one pathologist. HER2-ultralow prevalence ranged from 43% to 45% per pathologist, with 57% overall interpathologist concordance. Samples with no observable staining comprised most of the concordant cases. Treatment patterns were similar between HER2-ultralow and no-staining groups; however, time to treatment failure (TTF) varied between groups across lines of therapy (LOT). In HR- positive cohorts, median TTF for LOT1 was 7.73 months in patients with HER2-ultralow versus 9.43 months with no observable IHC staining. In HR- negative cohorts, median TTF was 5.00 months for HER2-ultralow versus 3.17 months with no observable IHC staining. Similar TTF trends were observed in LOT2-LOT3. Approximately three in five samples originally classified as IHC 0 met HER2-ultralow criteria, suggesting many patients may benefit from HER2-directed therapy if reclassified. These findings highlight potential challenges of identifying HER2-ultralow expression and suggest the need for enhanced pathologist training, adherence to best practices, and integration of digital pathology and artificial intelligence solutions. Trial Registration: ClinicalTrials.gov_identifier: NCT03734029.

PMID:
42340017
Bibliographic data and abstract were imported from PubMed on 24 Jun 2026.

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