The guidelines for establishing HER2 status for patients with breast cancer with unclear results on initial testing have been finetuned in an update that the American Society of Clinical Oncology and the College of American Pathologists recently issued.
Antonio C. Wolff, MD
The guidelines for establishing HER2 status for patients with breast cancer with unclear results on initial testing have been fine-tuned in an update that the American Society of Clinical Oncology (ASCO) and the College of American Pathologists (CAP) recently issued.
ASCO and CAP first established HER2 testing guidelines in 2007 and published an update in 2013. The 2018 Focused Update addresses uncommon clinical scenarios and seeks to explain how to handle HER2 testing results that are borderline, or “equivocal,” as previously defined in the guidelines,1 by immunohistochemistry (IHC) or in situ hybridization (ISH) (Table 1).2
An expert panel consisting of members of both organizations revised the diagnostic approach for patients who fall into groups categorized as 2, 3, and 4 by ISH testing (Table 21) to include more rigorous interpretation criteria for dual-probe ISH testing, and require an accompanying IHC review. The combined interpretation of the ISH and IHC assays is meant to improve the accuracy of HER2 status designation.
Only an estimated 5% of specimens will fall into the equivocal classification. Most specimens, about 95%, will be clearly HER2 positive (group 1) or clearly HER2 negative (group 5) and those will generally only require ISH or IHC testing.1
Although current outcomes data from clinical trials are of “limited statistical power,” Antonio C. Wolff, MD, professor of oncology at Johns Hopkins University School of Medicine and co-chair of the expert panel, said the goal was to better define the expected prognostic and predicted behavior of specimens tested by a dual-probe ISH that falls into one of the equivocal HER2 groups.
During an interview on the ASCO Guidelines Podcast Series episode, “HER2 Testing in Breast Cancer Guidelines,” Wolff said several labs and clinical investigators have reported on the practical implications of the 2013 guideline update and the observed frequency of equivocal cases.
M. Elizabeth H. Hammond, MD, emeritus professor of pathology at the University of Utah School of Medicine in Salt Lake City and co-chair of the expert panel, said in an interview with OncologyLive® that the guidelines are updated whenever new evidence becomes available or if there is confusion about the previous recommendations. In this case, a letter to the editor published by the Journal of Clinical Oncology led the panel to clear up some confusion regarding categorization of specimens that were neither clearly negative nor positive.2The panel has changed a recommendation regarding repeat HER2 testing of a surgical specimen. If the initial core biopsy is negative, a repeat test on the excision specimen is now optional rather than mandatory. Hammond said that clinical evidence shows the result is unlikely to change with a second test and may result in misdiagnosis.
“If you do a lot of repeat testing on the same sample, statistically what you do is increase the [odds] that you will have the wrong answer just by chance,” she said. “Repeat testing, in and of itself, is not a good idea.The panel also recommends that the same institution should perform the concomitant review to ensure “parallel interpretation and quality of the 2 assays.” Wolff said that very few group 2 specimens will be IHC 3+ and so will not be confirmed as HER2 positive. Most group 4 specimens will be confirmed HER2 negative without the need for additional testing using alternative probes, he added.
“Group 3 specimens will produce mixed results, with a small number of cases shown to be amplified and a larger number of cases not shown to be amplified,” he said. “So, for these dual-probe ISH group 3 cases, the panel allows specimens that are IHC 2+ to be considered HER2 positive.”
Hammond said clinicians dislike terms like “equivocal” to describe IHC 2+ results because they don’t know how to treat the patient.
“What we’re trying to do with this new guideline is make it so that we don’t use those words very often,” she added. “It’ll be vanishingly rare that we would use those words because now we can sort those cases into either positive or negative [categories].”
The updated guideline reverts to the FDA-approved definition of IHC 2+: invasive breast cancer with weak to moderate complete membrane staining observed in >10% of tumor cells.
If the specimen is assessed as group 3 with ISH testing and IHC analysis has not yet been performed, subsequent IHC tests should be conducted on sections from the same tissue sample. If the IHC result is 2+, an additional “observer” who is blinded to the previous ISH result should recount ISH. That person should count at least 20 cells, including the area of invasive cancer with IHC 2+ staining. If the review changes the ISH category, “the result should be adjudicated per internal procedures to define the final category,” the panel wrote.
Less common clinical results observed when performing dual-probe ISH testing are addressed by the recommended formulations in the new guidelines. When the specimen has a HER2/chromosome enumeration probe 17 (CEP 17) ratio of ≥2.0 but the average HER2 signals/ cell is <4.0 (group 2), the guidelines now call for additional IHC testing using sections from the previously used tissue sample. The treatment team should review slides from both ISH and IHC together to determine which areas to score by ISH. If the count remains an average of <4.0 HER2 signals/cell and HER2/CEP17 ratio of ≥2.0 after review by the blinded observer, or the IHC result is 0 or 1+, the diagnosis is HER2 negative.The expert panel added that the evidence on the efficacy of HER2-targeted therapy in this small subset of cases is limited. Data from the NRG B-47 trial (NCT01275677) showed that patients with tumors that do not have gene amplification and that are classified as 1+ or 2+ by IHC do not benefit from treatment with adjuvant trastuzumab (Herceptin).
Similar to the clinical results described above, specimens with average of ≥6.0 HER2 signals/ cell with a HER2/CEP17 ratio of <2.0 were previously defined as ISH positive, but now call for more testing. Further investigation is performed using the same process as for <4.0 HER2 signals/ cell and HER2/CEP17 ratio of ≥2.0 specimens. If the HER2/CEP17 ratio remains unchanged, the specimen is considered HER2 positive. If the IHC result is 0 or 1+, the sample is defined as HER2 negative.
The panel added that efficacy data for HER2- targeted therapy in patients with a HER2 ratio of <2.0 in the absence of protein overexpression are limited because these patients were ineligible for the first generation of adjuvant trastuzumab trials.
Although acknowledging that several singleprobe assays have regulatory approval in many places, the panel recommends the use of dualprobe instead of single-probe ISH assays.
“These actions, we hope, will reduce the already small number of cases that have HER2 test results that are unresolved, will reduce the frequency of alternative-probe testing, and will remind us all of the importance of integrating the information provided by both types of assay platforms in difficult cases as we expect these results to be concordant in the vast majority of cases, when expertly performed,” Wolff said.
Hammond added that, guidelines aside, the key to classifying and treating patients is communication among the care team.
“What we’re trying to recommend, like we have since 2007, is that the most effective way of getting the right answer for the patient is for the pathologist and the clinician to talk together about the case,” she said. “If you do that, most of the time the situation will get resolved.”