Jacob Pozin
Reid Shaw
Jennifer WeissForecasting and modeling is an inherently imprecise practice. However, the goal is to make predictions that are useful and as accurate as possible. Dichotomization, which is common in medicine, is universally considered a poor practice in modeling. Dichotomization leads to the loss of information, loss of effect size and power, spurious statistical significance, potential to overlook non-linear relationships, and loss of measurement reliability. (1–3) Despite this, it is perceived to ease interpretability, simplify analysis by reducing complexity, or allow for comparisons between multiple groups. However, few physicians would argue that a hemoglobin A1c of 6.5% is equivalent to 12% – despite both values meeting the criteria for diabetes. (4) When a continuous variable is dichotomized, the information contained in the specific values vanishes. In contrast, an individual with an A1c of 6.4% is unlikely to have significantly different outcomes than that of an individual with an A1c of 6.5%. However, with dichotomization, they are now viewed as distinct entities – decreasing the effect size and power. Although clinicians are unlikely to treat these scenarios similarly, problems arise when the dichotomized variable is less well understood and reporting on disease status is not standardized – as is the case of HER2-low in breast cancer.
Amplification of the HER2 gene is present in approximately 15% of early-stage breast cancer and is associated with an aggressive phenotype and increased risk of disease recurrence. Fortunately, there are HER2-targeting agents, such as trastuzumab, a humanized monoclonal antibody that improve overall survival in this patient cohort. (5) This landmark randomized control trial (RCT) used immunohistochemistry (IHC) to identify patients who may benefit from trastuzumab. (6) IHC separates HER2 into semi-quantitative categories: positive (3+), equivocal (2+), and negative (1+ and 0). (7) Although, HER2-positive tumors are most likely to respond to HER2-targeting therapy, the post-hoc analysis demonstrated that certain HER2-negative patients derived benefit from trastuzumab – opening the door for a new subset of patients termed HER2-low. (8) HER2-low is defined as IHC 1+ or 2+ with no amplification of HER2 by fluorescent in situ hybridization, a costly, but more accurate method of assessing HER2 status. However, when assessed in an RCT, trastuzumab failed to demonstrate a survival benefit in HER2-low breast cancer. (9) In support of these findings, multiple studies demonstrate that the HER2-low classification is not a reproducibly defined subtype of breast cancer with any distinct prognostic implication. (10–14) Despite these contradicting results, the drug-antibody conjugate, trastuzumab deruxtecan, showed improved overall survival in the HER2-low cohort. (15) It even demonstrated an objective response rate of 30% in HER2-negative (IHC 0) metastatic breast cancer. (16) Although the debate surrounding HER2-low as a distinct entity continues, the 2023 ASCO-College of American Pathologists encourages the reporting comment of IHC 0 versus 1+ to ensure identification of patients eligible for trastuzumab deruxtecan. These guidelines indicate the differentiation in IHC classification of 0 and 1+ is based on the detection of faint or barely perceptible incomplete membrane staining of amplified HER-2 in less or greater than 10% of tumor cells, respectively. (17) However, one survey of the College of American Pathologists shows that approximately 20% of cases read by laboratories generate results with less than 70% concordance for IHC HER2 score 0 versus 1+ (10). Despite this discrepancy, DESTINY-Breast06 is currently investigating trastuzumab deruxtecan in HER2-low and HER2-ultralow (IHC >0 <1+ expression). The results from the DESTINY-Breast06 trial presented at ASCO 2024 revealed that both HER2 low and ultralow cancers had similarly increased PFS and objective response rates (18). Although the DESTINY studies utilize a novel IHC assay to assess HER2 status, it is not clear if this test has less discordance than prior assays. To address this problem, quantitative measurements of HER2 are being developed that utilize mass spectrometry. (19) These methods of multiple reaction monitoring mass-spectrometry, which evaluate formalin-fixed-paraffin-embedded breast cancer tissues, have been found to both offer numerical quantification of HER2 levels that correlates well with established IHC classification, even at low HER2 expression levels (20, 21), as well as possibly distinguish between HER2-positive and HER2-negative samples with high specificity and sensitivity (22). Mass spectrometry identification of HER2 levels has also been shown to serve as a prognostic method in determining which patients will benefit from trastuzumab treatment. Specifically, higher HER2 levels (threshold >2200 amol/µg) were significantly associated with longer disease-free survival and overall survival in an adjuvant setting, as well longer overall survival in a metastatic setting (23). However, for quantitative HER2 methods to have clinical utility, they would need to be validated in trials studying whether they are predictive of benefit from HER2 targeted therapies in patients with HER2-positive, HER2-low, and HER2-ultralow breast cancer (24). Currently, we do not report ultralow HER2, however with HER2-ultralow showing clinical significance, it likely will change how we report HER2 status. For this reason, we need more standardized procedures, guidelines, and specialized training for pathologists in order to more accurately assess and report HER2-low and ultralow scores (25). Potentially, artificial intelligence IHC quantifier software will be used to consistently and accurately score HER2 expression (26).
Although the aim of this essay is not to settle the debate surrounding the legitimacy of HER2-low or HER2-ultralow as distinct biological entities, there are more appropriate and statistically rigorous means of evaluating HER2-targeted therapies. As evidenced by multiple prior studies, the answer is not to arbitrarily bin variables. Rather, HER2 expression should be kept as a continuous variable, with different treatment modalities based on a spectrum of HER2 expression. Treating HER2 as a continuous variable will likely decrease the number of patients needed in a clinical trial. (27) Furthermore, instead of spurious odds ratios determined by arbitrary cut points, the data may be presented as a simple line graph and confidence bands. (27) As the landscape continues to develop surrounding the clinical significant of HER2 status, newer, more standardized modalities to accurately evaluate and report its expression are increasingly needed to help individualize patient care and improve outcomes.
Author contributions
JP: Conceptualization, Data curation, Investigation, Methodology, Resources, Validation, Visualization, Writing – original draft, Writing – review & editing. RS: Conceptualization, Data curation, Investigation, Methodology, Resources, Validation, Visualization, Writing – original draft, Writing – review & editing. JW: Project administration, Resources, Supervision, Writing – original draft, Writing – review & editing.
Funding
The author(s) declare that no financial support was received for the research and/or publication of this article.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Publisher’s note
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References
1. MacCallum RC, Zhang S, Preacher KJ, Rucker DD. On the practice of dichotomization of quantitative variables. Psychol Methods. (2002) 7(1):19–40. doi: 10.1037/1082-989X
2. Van Walraven C, Hart RG. Leave ’em alone - Why continuous variables should be analyzed as such. Neuroepidemiology. (2008) 30(3):138–9. doi: 10.1159/000126908
3. Royston P, Altman DG, Sauerbrei W. Dichotomizing continuous predictors in multiple regression: A bad idea. Stat Med. (2006) 25(1):127–41. doi: 10.1002/sim.2331
4. American Diabetes Association. Standards of medical care in diabetes-2011. Diabetes Care. (2011) 34 Suppl 1(Suppl 1):S11–61. doi: 10.2337/dc11-S011
5. Bradley R, Braybrooke J, Gray R, Hills R, Zulian L, Peto R, et al. Trastuzumab for early-stage, HER2-positive breast cancer: a meta-analysis of 13 864 women in seven randomised trials. Lancet Oncol. (2021) 22(8):1139–50. doi: 10.1016/S1470-2045(21)00288-6
6. Romond EH, Perez EA, Bryant J, Suman V, Geyer C, Davidson N, et al. Trastuzumab plus Adjuvant Chemotherapy for Operable HER2-Positive Breast Cancer. New Engl J Med. (2005) 353(16):1673–84. doi: 10.1056/nejmoa052122
7. Wolff AC, McShane LM, Hammond MEH, Harvey B, Mangu P, Bartlett J, et al. Human epidermal growth factor receptor 2 testing in breast cancer: American Society of Clinical Oncology/College of American Pathologists Clinical Practice Guideline Focused Update. Arch Pathol Lab Med. (2018) 142(11):1364–82. doi: 10.5858/arpa.2018-0902-SA
8. Perez EA, Reinholz MM, Hillman DW, Tenner K, Schroeder M, Davidson N, et al. HER2 and chromosome 17 effect on patient outcome in the N9831 adjuvant trastuzumab trial. J Clin Oncol. (2010) 28(28):4307–15. doi: 10.1200/JCO.2009.26.2154
9. Fehrenbacher L, Cecchini RS, Geyer CE, Rastogi P, Constantino J, Atkins J, et al. NSABP B-47/NRG oncology phase III randomized trial comparing adjuvant chemotherapy with or without trastuzumab in high-risk invasive breast cancer negative for HER2 by FISH and with IHC 1+ or 2+. J Clin Oncol. (2020) 38(5):444–53. doi: 10.1200/JCO.19.01455
10. Fernandez AI, Liu M, Bellizzi A, Brock J, Fadare O, Hanley K, et al. Examination of Low ERBB2 Protein Expression in Breast Cancer Tissue. JAMA Oncol. (2022) 8(4):1–4. doi: 10.1001/jamaoncol.2021.7239
11. Miglietta F, Griguolo G, Bottosso M, Giarratano T, Lo Mele M, Fassan M, et al. Evolution of HER2-low expression from primary to recurrent breast cancer. NPJ Breast Cancer. (2021) 7(1):137. doi: 10.1038/s41523-021-00343-4
12. Tarantino P, Jin Q, Tayob N, Jeselsohn R, Schnitt S, Vincuilla J, et al. Prognostic and Biologic Significance of ERBB2-Low Expression in Early-Stage Breast Cancer. JAMA Oncol. (2022) 8(8):1177–83. doi: 10.1001/jamaoncol.2022.2286
13. Hein A, Hartkopf AD, Emons J, Lux M, Volz B, Taran F, et al. Prognostic effect of low-level HER2 expression in patients with clinically negative HER2 status. Eur J Cancer. (2021) 155:1–12. doi: 10.1016/j.ejca.2021.06.033
14. Schettini F, Chic N, Brasó-Maristany F, Pare L, Pascual T, Conte B, et al. Clinical, pathological, and PAM50 gene expression features of HER2-low breast cancer. NPJ Breast Cancer. (2021) 7(1):1. doi: 10.1038/s41523-020-00208-2
15. Modi S, Jacot W, Yamashita T, Sohn J, Vidal M, Tokunaga E, et al. Trastuzumab Deruxtecan in Previously Treated HER2-Low Advanced Breast Cancer. New Eng J Med. (2022) 387(1):9–20. doi: 10.1056/nejmoa2203690
16. Diéras V, Deluche E, Lusque A, Pistilli B, Bachelot T, Pierga J, et al. Abstract PD8-02: Trastuzumab deruxtecan (T-DXd) for advanced breast cancer patients (ABC), regardless HER2 status: A phase II study with biomarkers analysis (DAISY). Cancer Res. (2022) 82(4). doi: 10.1158/1538-7445.sabcs21-pd8-02
17. Wolff AC, Somer MR, Dowsett M, Hammond MEH. Human Epidermal growth factor receptor 2 testing in breast cancer : asco – college of american pathologists guideline update. J Clin Oncol. (2023) 41(22):3867–72. doi: 10.1200/JCO.22.02864
18. Curigliano G, Hu X, Dent RA, Yonemori K, Barrios C, O’Shaughnessy J, et al. Trastuzumab deruxtecan (T-DXd) vs physician’s choice of chemotherapy (TPC) in patients (pts) with hormone receptor-positive (HR+), human epidermal growth factor receptor 2 (HER2)-low or HER2-ultralow metastatic breast cancer (mBC) with prior endocrine therapy (ET): Primary results from DESTINY-Breast06 (DB-06) [abstract]. ASCO. (2024).
19. Moutafi M, Robbins CJ, Yaghoobi V, Fernandez A, Martinez-Morilla S, Xirou V, et al. Quantitative measurement of HER2 expression to subclassify ERBB2 unamplified breast cancer. Lab Invest. (2022) 102(10):1101–8. doi: 10.1038/s41374-022-00804-9
20. Kennedy JJ, Whiteaker JR, Kennedy LC, Bosch D, Lerch M, Schoenherr R, et al. Quantification of Human Epidermal Growth Factor Receptor 2 by Immunopeptide Enrichment and Targeted Mass Spectrometry in Formalin-Fixed Paraffin-Embedded and Frozen Breast Cancer Tissues. Clin Chem. (2021) 67(7):1008–18. doi: 10.1093/clinchem/hvab047
21. Do M, Kim H, Yeo I, Lee J, Park I, Ryu H, et al. Clinical application of multiple reaction monitoring-mass spectrometry to human epidermal growth factor receptor 2 measurements as a potential diagnostic tool for breast cancer therapy. Clin Chem. (2020) 66(10):1339–48. doi: 10.1093/clinchem/hvaa178
22. Ivanova M, Porta FM, D'Ercole M, Pescia C, Sajjadi E, Cursano G, et al. Standardized pathology report for HER2 testing in compliance with 2023 ASCO/CAP updates and 2023 ESMO consensus statements on HER2-low breast cancer. Virchows Arch Publsihed Online. (2023) 484(1):3–14. doi: 10.1007/s00428-023-03656-w
23. Nuciforo P, Thyparambil S, Aura C, Garrido-Castro A, Vilaro M, Peg V, et al. High HER2 protein levels correlate with increased survival in breast cancer patients treated with anti-HER2 therapy. Mol Oncol. (2016) 10(1):138–47. doi: 10.1016/j.molonc.2015.09.002
24. Park J, Oh HJ, Han D, Wang J, Park I, Ryu H, et al. Parallel Reaction Monitoring-Mass Spectrometry (PRM-MS)-Based Targeted Proteomic Surrogates for Intrinsic Subtypes in Breast Cancer: Comparative Analysis with Immunohistochemical Phenotypes. J Proteome Res. (2020) 19(7):2643–53. doi: 10.1021/acs.jproteome.9b00490
25. Frey P, Mamilos A, Minin E, Banisch R, Günther S, Schmidt C, et al. AI-based HER2-low IHC scoring in breast cancer across multiple sites, clones, and scanners. J Clin Oncol. (2023) 41(16_suppl):516–6. doi: 10.1200/jco.2023.41.16_suppl.516
26. Zhao LP, Kolonel LN. Efficiency loss from categorizing quantitative exposures into qualitative exposures in case-control studies. Am J Epidemiol. (1992) 136(4):464–74. doi: 10.1093/oxfordjournals.aje.a116520
Keywords: HER2, breast cancer, trastuzumab, IHC, dichotomization, mass spectrometry
Citation: Pozin J, Shaw R and Weiss J (2025) Breaking the dichotomy of HER2: highlighting the flaws of simplifying variables. Front. Oncol. 15:1494417. doi: 10.3389/fonc.2025.1494417
Received: 10 September 2024; Accepted: 21 April 2025;
Published: 09 May 2025.
Edited by:
Jo Chien, University of California, San Francisco, United StatesReviewed by:
Silvana Marcia Bruschi Kelles, NATS UNIMED-BH, BrazilCopyright © 2025 Pozin, Shaw and Weiss. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Jacob Pozin, amFjb2IucG96aW5AbHVocy5vcmc=