HER2 Classification Evolution

The discovery that approximately 20-25% of breast cancers overexpress human epidermal growth factor receptor 2 (HER2) transformed breast cancer from a single disease into molecularly defined subtypes. Dennis Slamon's pivotal work in the late 1980s demonstrated that HER2 amplification was associated with aggressive disease and poor prognosis. Initial testing relied on immunohistochemistry (IHC), which categorized tumors as 0, 1+, 2+, or 3+. However, IHC alone was insufficiently reliable for treatment decisions, with significant interlaboratory variability particularly for the 2+ equivocal category. The development of fluorescence in situ hybridization (FISH) provided a more objective measure of HER2 gene amplification to complement IHC.

The ASCO/CAP HER2 testing guidelines published in 2007 established the first standardized framework for HER2 assessment, defining IHC 3+ or FISH-amplified as HER2-positive and eligible for trastuzumab therapy. These guidelines addressed the critical problem of testing variability that had led to approximately 20% of HER2 results being inaccurate. The 2013 update refined the equivocal IHC 2+ category and redefined FISH positivity thresholds, responding to evidence that the previous criteria were missing some amplified cases and including some non-amplified ones. These standardization efforts were directly driven by the therapeutic implications—trastuzumab showed dramatic survival benefits in HER2-positive breast cancer (HERA trial, BCIRG-006), making accurate testing a matter of life and death.

The paradigm shift came with the DESTINY-Breast04 trial, published in 2022, which demonstrated that trastuzumab deruxtecan (T-DXd, Enhertu) provided significant survival benefit in HER2-low breast cancer—tumors previously classified as IHC 1+ or IHC 2+/FISH-negative that had been considered HER2-negative and ineligible for anti-HER2 therapy. This trial effectively rendered the traditional binary HER2-positive/negative classification obsolete, creating a new 'HER2-low' category comprising approximately 55% of all breast cancers. Pathologists were suddenly required to reliably distinguish IHC 0 from IHC 1+, a distinction that had previously been considered clinically irrelevant and was poorly reproducible. The DESTINY-Breast06 trial further extended T-DXd benefit to HER2-ultralow (faint incomplete staining) tumors.

The 2023 ASCO/CAP HER2 testing guideline update explicitly addressed the HER2-low category, recommending that pathology reports specify the IHC score (0, 1+, 2+, 3+) rather than simply reporting positive or negative. ESMO guidelines now include HER2-low as a clinically actionable category with T-DXd as a recommended treatment option. The NCCN breast cancer guidelines were updated to include HER2-low-directed therapy in the treatment algorithm. These changes placed enormous new demands on pathology laboratories to ensure reproducible IHC scoring at the low end of the HER2 expression spectrum, driving interest in digital pathology and AI-assisted scoring.

The HER2-low revolution has fundamentally reshaped breast cancer pathology and treatment. Pathology laboratories worldwide are recalibrating their IHC scoring practices, with increasing use of digital pathology and AI-assisted quantification to improve reproducibility at low expression levels. The distinction between IHC 0 and IHC 1+ has become the new critical threshold, yet studies show interobserver agreement for this distinction remains poor (kappa 0.40-0.50). Research is exploring whether continuous HER2 expression quantification using mass spectrometry or mRNA-based assays could replace categorical IHC scoring. The concept of HER2-ultralow further challenges the notion of a true HER2-zero tumor. The paradigm has shifted from 'does this tumor have enough HER2 to be targeted?' to 'does this tumor have any HER2 at all?'