ERBB2/HER2 Expression in a Real-World Oncology Cohort:
A Retrospective Observational Study
Dr. Mohammad Imam Ather *
*Correspondence to: Dr. Mohammad Imam Ather, Clinical Oncology and Hematology.
Copyright
© 2026 Dr. Mohammad Imam Ather is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Received: 12 October 2026
Published: 01 November 2026
DOI: https://doi.org/10.5281/zenodo.23910946
Background: HER2, encoded by the ERBB2 gene, is an important predictive biomarker in breast cancer and other solid tumors. HER2 assessment by immunohistochemistry (IHC), in situ hybridization (ISH), and molecular testing guides therapeutic decisions. The increasing use of antibody-drug conjugates has also increased the importance of accurately identifying lower levels of HER2 expression.¹
Methods: We retrospectively analyzed an institutional ERBB2/HER2 testing dataset containing 552 test records from 518 unique patients. The study was conducted over a period of 5 years and 2 months, from July 2021 to August 2026. Testing included IHC, fluorescence in situ hybridization (FISH), and next-generation sequencing (NGS). Demographic characteristics, tumor type, HER2 IHC status, and ERBB2 molecular findings were evaluated.
Results: IHC was the most frequently used testing method, accounting for 444 (80.4%) records, followed by NGS in 96 (17.4%) and FISH in 12 (2.2%). Breast cancer was the predominant diagnosis, comprising 341 (61.8%) records. Among 333 breast cancer IHC records, 176 (52.9%) were IHC 0, 21 (6.3%) IHC 1+, 25 (7.5%) IHC 2+, and 111 (33.3%) IHC 3+. The mean age was similar between IHC 3+ and IHC 0–2+ groups (56.3 vs 56.4 years; p=0.967). No significant association was observed between IHC 3+ status and tumor stage (p=0.678). ERBB2 alterations were detected in 5 of 96 NGS records (5.2%).
Conclusion: This real-world cohort demonstrates substantial heterogeneity in HER2 expression. Approximately one-third of breast cancer IHC records showed strong HER2 expression, while lower expression categories were also observed. NGS identified ERBB2 alterations in a subset of tested tumors. Accurate HER2 assessment and standardized reporting remain important for biomarker-directed treatment.
Introduction
HER2 is a transmembrane receptor encoded by the ERBB2 gene and plays an important role in tumor cell proliferation and survival. HER2 amplification or protein overexpression is particularly important in breast cancer because it identifies patients who may benefit from HER2-directed therapies.¹. HER2 status is primarily assessed using IHC and ISH. Current ASCO–CAP recommendations classify IHC 3+ tumors as HER2-positive, while IHC 2+ tumors require appropriate ISH evaluation.¹,². The therapeutic relevance of lower HER2 expression has increased with the development of antibody-drug conjugates. The DESTINY-Breast04 trial evaluated trastuzumab deruxtecan in previously treated metastatic breast cancer with HER2-low expression, defined as IHC 1+ or IHC 2+/ISH-negative disease, and demonstrated improved clinical outcomes compared with physician's-choice chemotherapy.³. Therefore, accurate differentiation between IHC 0 and IHC 1+ has become increasingly important. The 2023 ASCO–CAP update specifically emphasizes careful reporting of the IHC score and distinction between IHC 0 and IHC 1+ results.¹
Materials and Methods
This retrospective observational study analyzed an institutional ERBB2/HER2 testing database containing 552 test records from 518 unique patients. The study was conducted over a period of 5 years and 2 months, from July 2021 to August 2026.Testing included IHC, FISH, and NGS. IHC results were categorized as 0, 1+, 2+, or 3+. Breast cancer cases were analyzed separately for HER2 expression and its relationship with age and disease stage.
ERBB2 alterations identified by NGS were recorded descriptively. Continuous variables were summarized using mean and standard deviation, while categorical variables were summarized using frequencies and percentages. A p value <0.05 was considered statistically significant.
Results
The study included 552 test records from 518 unique patients. The mean age of the overall cohort was 59.1 ± 11.8 years, with females comprising 82.1% of records.
IHC was the predominant testing method, representing 80.4% of all records. NGS and FISH accounted for 17.4% and 2.2%, respectively.
Breast cancer was the most common malignancy, accounting for 341 records from 326 unique patients. Among the 333 breast cancer IHC records, HER2 expression was IHC 0 in 176 cases (52.9%), IHC 1+ in 21 (6.3%), IHC 2+ in 25 (7.5%), and IHC 3+ in 111 (33.3%).
The mean age of patients with IHC 3+ disease was 56.3 years compared with 56.4 years among patients with IHC 0–2+ disease, with no significant difference (p=0.967). HER2 IHC 3+ expression was observed across multiple disease stages. No statistically significant association was identified between HER2 IHC 3+ status and tumor stage (p=0.678). Among 96 NGS records, ERBB2 alterations were detected in 5 cases (5.2%), while 91 cases (94.8%) were reported as ERBB2 not detected.
Discussion
This study provides a real-world assessment of HER2/ERBB2 testing in an institutional oncology population. Breast cancer constituted the majority of cases, and IHC was the principal testing modality.
HER2 IHC 3+ expression was identified in 33.3% of breast cancer IHC records. Importantly, 13.8% of cases showed IHC 1+ or IHC 2+ expression. These lower-expression categories have gained clinical importance following the introduction of HER2-directed antibody-drug conjugates.¹,³. The ASCO–CAP guideline recommends continuing to report the actual HER2 IHC score and emphasizes careful distinction between IHC 0 and IHC 1+, because this distinction may influence eligibility for certain HER2-directed therapies in metastatic disease.¹. In our cohort, no significant relationship was observed between HER2 IHC 3+ status and either age or disease stage. These findings should be considered exploratory because of the retrospective design and limited clinical variables. ERBB2 alterations were identified in 5.2% of NGS-tested records. Molecular ERBB2 alterations and HER2 protein expression represent different biological parameters; therefore, NGS results should be interpreted together with histopathologic and IHC findings rather than considered interchangeable with HER2 positivity.
Conclusion
This real-world analysis demonstrates substantial heterogeneity in HER2/ERBB2 findings. Breast cancer was the predominant tumor type, with HER2 IHC 3+ expression identified in approximately one-third of breast cancer IHC records. Lower HER2 expression was also observed, emphasizing the importance of accurate IHC scoring, particularly differentiation between IHC 0 and IHC 1+. ERBB2 alterations were identified in a subset of NGS-tested tumors, supporting the complementary role of molecular profiling. Further studies incorporating treatment and survival outcomes are required to determine the clinical impact of these findings.
References