Dr Rimm on the Effect of Inaccurate HER2 IHC Testing for Patients With Breast Cancer

“Inaccurate IHC testing doesn’t affect cancer diagnosis; it affects subclassification of the cancer. We all know that the diagnosis is breast cancer, and so this test is not a diagnostic test; it’s what's called a companion diagnostic test.”

David Rimm, MD, PhD, Anthony N. Brady Professor of Pathology, professor, medicine, Medical Oncology, Yale School of Medicine; director, Physician Scientist Training Program, Pathology Research, director, Tissue Microarray Facility, director, Yale Pathology Tissue Services, Pathology, Yale Cancer Center, discusses the effect of inaccurate HER2 immunohistochemistry (IHC) testing on the subclassification of patients with breast cancer.

Inaccurate IHC testing does not directly affect the diagnosis of cancer but rather its subclassification, which plays a critical role in guiding treatment decisions, Rimm begins. Although breast cancer is diagnosed independently, the IHC test for HER2 is a companion diagnostic test, according to Rimm. This means it is specifically tied to determining eligibility for HER2-targeted therapies, such as fam-trastuzumab deruxtecan-nxki (T-DXd; Enhertu), which has significantly improved patient outcomes, he explains.

However, the accuracy of the HER2 IHC test remains a concern, Rimm continues, saying that patients classified as HER2 1+ whoare actually HER2 0 may receive T-DXd unnecessarily and achieve no benefit from the drug. Conversely, some patients classified as HER2 0 might have low levels of HER2 expression that could make them eligible for treatment with HER2-directed therapies but are excluded because the test did not detect sufficient expression, he reports. This misclassification limits the effectiveness of personalized treatment and prevents some patients from accessing potentially beneficial therapies, Rimm emphasizes.

The regulation of HER2 IHC tests further complicates the issue, Rimm expands. Unlike other IHC stains used as binary diagnostic tools to confirm cancer diagnoses, since the HER2 IHC test functions as a companion diagnostic, it is regulated by the FDA as a device rather than a stain, he says. Other IHC stains fall under lower regulatory classifications due to their confirmatory nature, he notes. This distinction highlights the unique role and challenges of HER2 IHC testing, as it directly affects therapeutic decision-making rather than serving as a simple diagnostic aid, Rimm concludes.

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David Rimm, MD, PhD, discusses the effect of inaccurate HER2 immunohistochemistry testing on the subclassification of patients with breast cancer.

Dr Rimm on the Utility and Shortcomings of HER2 IHC Testing in Breast Cancer

“Not only is it impossible for pathologists to tell a [HER2] 0+ from [a] 1+, but the machines that make the stain can [also be a] variable as well and can add to the problem of telling 0 plus from 1 plus.”

David Rimm, MD, PhD, Anthony N. Brady Professor of Pathology, professor, medicine, Medical Oncology, Yale School of Medicine; director, Physician Scientist Training Program, Pathology Research, director, Tissue Microarray Facility, director, Yale Pathology Tissue Services, Pathology, Yale Cancer Center, discusses current HER2 immunohistochemistry (IHC) assays that are used in breast cancer disease classification.

One key challenge with diagnostic stains, particularly companion diagnostics, lies in the need for standardization between laboratories, Rimm begins. Historically, there has been no effective way to standardize results across laboratories, leading to significant variability, according to Rimm. The assumption that using the same autostainer and reagents yields consistent results has been debunked, he reports. Recent findings demonstrated that staining intensity could differ up to 4- or 5-fold between labs, even those using identical equipment and reagents, Rimm emphasizes. This inconsistency is especially problematic for HER2 IHC testing, particularly in the low-expression category, he says. A HER2 result categorized as “low” or 1+ in one lab might be undetectable, or HER2 0+, in another, leading to discrepancies, he notes.

To address this issue, a new quality control tool has been developed, he continues. This slide, which includes antigens bound to glass beads, provides a standardized measure of HER2 detection that can be used to produce consistent results across laboratories, he says. By including reference spots representing various antigen concentrations, laboratories can assess the performance of their staining systems, Rimm explains. For instance, if a lab fails to detect a specific spot that corresponds to a known detection threshold, the tool signals the need to recalibrate or rerun the batch to ensure proper functionality, he notes.

Although this approach advances the accuracy of machine-based staining, it does not solve variability in pathologist interpretations, Rimm concludes, adding that addressing both machine and human factors remains critical to improving diagnostic accuracy.

David Rimm, MD, PhD, discusses current HER2 immunohistochemistry assays that are used in the management of breast cancer, and their shortcomings.

Dr Rimm on Limitations With IHC Testing in HER2-Low and HER2-Ultralow Breast Cancer

David Rimm, MD, PhD, Anthony N. Brady Professor of Pathology, professor, medicine, Medical Oncology, Yale School of Medicine; director, Physician Scientist Training Program, Pathology Research, director, Tissue Microarray Facility, director, Yale Pathology Tissue Services, Pathology, Yale Cancer Center, discusses the limitations of current immunohistochemistry (IHC) diagnostic kits for patients with HER2-low and HER2-ultralow breast cancer.

Researchers need to design diagnostic tests in a manner that optimizes pathologists’ ability to achieve reproducible results, Rimm begins, adding that the current tests do not meet this standard. In his view, the breast cancer field should move beyond relying on pathologists to subjectively “read” tests, and instead focus on precise measurements. Measurements, rather than subjective readings, are objective, standardized methodsof quantifying a target, such as providing the concentration of a biomarker on a slide in a defined unit, he states. This is an aspirational goal, and only a few laboratories globally are currently capable of such measurements, according to Rimm. However, Rimm believes that transitioning toward this model is essential for providing the best care to patients.

In fields like lung cancer, for instance, tests are used to measure EGFR expression levels to determine patient eligibility for TKIs, he continues. This testing approach relies on objective measurements rather than subjective readings by pathologists, he explains. Measurements allow for reproducibility and enable quality control, ensuring that the machines used are properly calibrated and functioning consistently, Rimm emphasizes.

Rimm believes that the future of IHC lies in such standardized measurements rather than subjective readings, but that achieving this change will require significant time. In the meantime, the oncology field must work to standardize current IHC reading methods by ensuring uniformity in the machines used, addressing one of the weakest points in the current system, he notes. Ultimately, future tests should be designed to be both reproducible and manageable for pathologists, Rimm concludes.

David Rimm, MD, discusses the current limitations of immunohistochemistry diagnostic kits for HER2-low and HER2-ultralow breast cancer.

David Rimm, MD, PhD,  discusses the limitations of current HER2 immunohistochemistry assays in breast cancer.

Dr Rimm on the Limitations of HER2 IHC Assays in Breast Cancer

David Rimm, MD, PhD, Anthony N. Brady Professor of Pathology, professor, medicine, Medical Oncology, Yale School of Medicine; director, Physician Scientist Training Program, Pathology Research, director, Tissue Microarray Facility, director, Yale Pathology Tissue Services, Pathology, Yale Cancer Center, 

discusses the limitations of current HER2 immunohistochemistry (IHC) assays

 in breast cancer, and how inaccurate testing results impact outcomes for patients with low HER2 expression.

Rimm begins by clarifying that the IHC test for HER2 is a companion diagnostic rather than a standard diagnostic test, meaning it is specifically used to determine eligibility for certain therapies. In this case, the companion diagnostic is primarily associated with the agent fam-trastuzumab deruxtecan-nxki (Enhertu; T-DXd), he adds. T-DXd is a highly effective drug that has significantly improved treatment outcomes for patients with HER2-low or HER2-positive cancers, Rimm states. However, the ability of this diagnostic test to accurately identify patients who have the potential to benefit from HER2-targeted therapies such as T-DXd remains a concern, he emphasizes.

The test categorizes HER2 expression levels, and patients are classified as HER2 0, 1+, 2+, or 3+. However, this grading system isn't always precise, Rimm notes. One major issue is that patients identified as 1+ may actually have no detectable HER2 expression, yet they might still be given the drug, which would provide little or no therapeutic benefit, Rimm explains. On the other hand, some patients classified as IHC 0 may have enough HER2 expression to benefit from the treatment but are excluded due to insufficient HER2 detection by the IHC test, he details. This diagnostic inaccuracy results in potentially effective therapy being withheld from patients who could respond well to it, Rimm concludes.

The limitations of the IHC test highlight a gap between diagnostic precision and the potential for therapeutic benefit, Rimm summarizes. As HER2-targeted therapies continue to expand, refining the accuracy of companion diagnostics becomes increasingly important. Ensuring that the right patients are selected for treatment using more reliable detection methods could further enhance outcomes and expand the reach of these agents.

Dr Rimm on Unmet Needs in Distinguishing Between HER2-Low and HER2-Ultralow Breast Cancer

David Rimm, MD, PhD, Anthony N. Brady Professor of Pathology, professor, medicine, Medical Oncology, Yale School of Medicine; director, Physician Scientist Training Program, Pathology Research, director, Tissue Microarray Facility, director, Yale Pathology Tissue Services, Pathology, Yale Cancer Center, discusses the primary challenges in distinguishing between HER2-low and HER2-ultralow breast cancer using current immunohistochemistry diagnostic kits.

The biggest challenge is that the current diagnostic kits for measuring HER2 levels operate outside the appropriate dynamic range, Rimm begins. Essentially, the current tests can differentiate between HER2 gene–amplified cases and non-amplified cases, but this involves detecting a high number of molecules, he reports. HER2-low cases involve fewer molecules, and the tests are not designed to detect HER2 at this lower range, Rimm emphasizes, adding that this may be compared with trying to weigh mice on a scale meant for elephants, leading to inaccurate results.

Studies have shown that pathologists struggle to accurately differentiate between HER2 0 and HER2 1+ disease, where HER2 0 is considered HER2-low disease and HER2 0 is considered HER2-negative disease, he continues. Despite extensive training and global comparisons, the accuracy of these tests remains at approximately 80%, which is insufficient for patient care, Rimm adds. In comparison, molecular tests for other diagnostics, such as 

 mutations in the realm of lung cancer, have a reproducibility of 99%, which is the standard that breast cancer researchers should aim for, he says.

Currently, pathologists are being asked to distinguish even finer gradations, such as HER2 half from HER2 0, which is not feasible given that distinguishing HER2 0 from HER2 1+ is already challenging, Rimm expands. This approach seems poorly thought out and unrealistic, likely driven by the financial motives of drug companies rather than practical diagnostic capabilities, he emphasizes. Consequently, the attempt to differentiate HER2 0 from HER2 1 or HER2 levels that are less than 1 but greater than 0, is impractical and undermines the field of pathology by imposing impossible tasks on diagnostic tests, Rimm concludes.

David Rimm, MD, PhD, discusses challenges in distinguishing between HER2-low and HER2-ultralow breast cancer using current diagnostic kits.

David Rimm, MD, PhD, Anthony N. Brady professor of Pathology, professor of Medicine (Medical Oncology), Yale School of Medicine, director, Yale Cancer Center Tissue Microarray Facility, Pathology, director, Yale Pathology Tissue Services, director, Physician Scientist Training Program, Pathology Research, Yale Cancer Center, discusses both the use and advantages of utilizing multiplex spatial proteomic profiling in breast cancer.

At the 2022 San Antonio Breast Cancer Symposium, Rimm presented on multiplex spatial proteomic profiling, which is a tissue analysis conducted through precise epitope colocalization, allowing for the observation of cellular functional states in view of the spatial organization of respective cells. This process is being used in breast cancer in an effort to discover new biomarkers by examining multiple proteins at one time while still preserving the spatial information of the tissue sample, Rimm begins. Instead of grinding up the specimen, investigators look at the specimen with a microscope and can observe a plethora of proteins in a single viewing, Rimm notes, adding that these proteins are observed for their expression levels and their localization to try to find biomarkers associated with various therapies. This process could be used to search for biomarkers associated with immunotherapy or those associated with targeted therapy, Rimm adds.

The key advantage to using multiplex spatial proteomic profiling is preserving the differences in the tissue, since there are multiple differences between respective cells, Rimm continues. Samples can include cells that are associated with response to therapy, cells that are associated with the tumor, and cells that are associated with immune reaction, Rimm says. When these different cells are grinded together, it becomes more challenging to draw conclusions without information about a specific cell’s origin, Rimm explains.

Overall, a lot of the topics addressed at the 2022 San Antonio Breast Cancer Symposium centered around methods to look at expression of the proteins in the tissue itself, measuring the amounts and localization of each protein in order to find biomarkers associated with responses to therapy, Rimm concludes.

David Rimm, MD, PhD, discusses both the use and advantages of utilizing multiplex spatial proteomic profiling in breast cancer.

Dr. Rimm on the Use of Multiplex Spatial Proteomic Profiling in Breast Cancer

	David Rimm MD, PhD, professor of pathology and medicine, director of pathology tissue services, Yale University School of Medicine, explains the benefits of quantitative immunofluorescence for the measurement of HER2 proteins. 

has twice as much dynamic range, and it is more reproducible, says Rimm.  The technology makes it easier to match patients with their most effective therapies. 

	Most immunohistochemistry (IHC) assays, which are traditionally used for the measurement of HER2 proteins, have a 10% to 20% false negative and false positive rate. Quantitative immunofluorescence is more accurate, sensitive, 

	This practice has not yet caught on with many pathologists yet, but Rimm believes the field will move into that direction once pathologists begin to see its value and it becomes standard practice. 

David Rimm MD, PhD, professor of pathology and medicine, director of pathology tissue services, Yale University School of Medicine, explains the benefits of quantitativecfor the measurement of HER2 proteins. 

David Rimm MD, PhD

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