Low level limbo in HER2 breast cancer

Karen Titus

August 2023—Seemingly channeling the inspiration of Magritte and his famous pipe, pathologists are painting a new picture of what has long been an everyday object in their own world: HER2.

To paraphrase the master: Ceci n’est pas facile.

For years, HER2 testing in breast cancer has seemed self-evident, ever since the HER2-targeted therapy trastuzumab and its companion diagnostic arrived on the scene a quarter of a century ago. Pathologists became comfortable using immunohistochemistry to identify 3+ cases and turning to in situ hybridization techniques to sort through less obvious ones.

But early last summer, a variant of the drug, trastuzumab-deruxtecan (T-DXd), shook up that routine. When researchers presented results from the Destiny-Breast04 study at the 2022 ASCO annual meeting, showing that T-DXd significantly improves survival in so-called HER2-low metastatic breast cancer, attendees responded with a minutes-long standing ovation.

They then returned from the meeting like evangelicals from the revival tent. “The quest started then,” says Shabnam Jaffer, MD, chair, Department of Pathology and Laboratory Medicine, Lenox Hill Hospital/Northwell Health, New York City, and professor of pathology, Zucker School of Medicine, Hofstra University. A case that was 1+ IHC or 2+ IHC/ISH-negative meant patients qualified for the antibody-drug conjugate. “So the oncologists were all very excited and motivated to start labeling which patients were HER2 low. They wanted to treat those patients, especially those who had failed other treatments.”

The motivation was stronger than the methods, as pathologists were quick to realize. It was no longer appropriate to lump the 0 and 1+ cases together as “negative,” but the assays that had worked well for identifying cases that were strongly HER2 positive were never meant to parse the particulars at lower levels.

ASCO and the CAP moved quickly as well, and this spring the steering committee published an update to the ASCO-CAP guideline on HER2 testing in breast cancer (Wolff AC, et al. Arch Pathol Lab Med. Published online June 7, 2023. doi:10.5858/arpa.2023-0950-SA).

“It was much anticipated, much awaited by pathologists,” says Dr. Jaffer, “because we were starting to get bombarded by requests and questions from our oncologists, asking about both current and historic specimens: By the way, that patient you signed out is metastatic—can you go back and let me know if it was HER2 low or not? Many institutions were in a flurry to figure it out.”

Prior to the Destiny-B04 trial, there was no incentive to provide granularity to these reports, she notes. Current methods didn’t make it easy to separate 0 and 1+ cases, nor was there a need to; it made sense for testing results to be perceived as binary. “But now that philosophy is changing as we see these new drugs work in patients who are not necessarily HER2 expressed or amplified,” Dr. Jaffer says.

The update, the authors make clear, is not a revision. The guideline remains in force. It does, however, offer five best practices to help pathologists and their clinical colleagues navigate the more immediate HER2 testing challenges raised by antibody-drug conjugates.

But these are only first steps in a field that everyone agrees could look completely different in the not-too-distant future, as pathologists decide what to look for, and how.

There’s also the question of what to call these cases. HER2 low is not a clinical category, though the term has become common parlance among physicians. With new ways to assess HER2 in development, as well as dozens of antibody-drug conjugates ready to pop out of the pipeline, “HER2 low” may be a placeholder, eventually giving pathologists the Adam-like responsibility of naming what lies before them.

Despite the locomotive rate at which this all seems to be evolving, the guideline is notable for what it doesn’t change, says Kimberly Allison, MD, an author on the update.

“We opted not to create a new reporting category of HER2 low,” says Dr. Allison, director of breast pathology, professor of pathology, and vice chair of education, Department of Pathology, Stanford University School of Medicine. Instead, the authors wanted to call attention to the difference it makes clinically when a case is scored as 0 versus 1+. They suggest pathologists use a report comment, or footnote, with proposed wording. “That was our way of explaining that patients who don’t have HER2 overexpression or amplification may be eligible for these other therapies,” she says.

“It’s pragmatic guidance to pathologists in terms of how to deal with HER2-low breast cancers and their categorization,” says Stuart Schnitt, MD, chief of breast oncologic pathology, Dana-Farber Brigham Cancer Center; associate director, Dana-Farber Cancer Institute-Brigham and Women’s Hospital Breast Oncology Program; and professor of pathology, Harvard Medical School. “There’s nothing radical that we need to do at this point,” but rather, continue to follow the 2018 ASCO-CAP guidelines, while noting the more focused emphasis on figuring out the difference between 0 and 1+ cases. (Dr. Schnitt is coauthor of an editorial published in the same issue as the update: Schnitt SJ, et al. Arch Pathol Lab Med. Published online June 7, 2023. doi:10.5858/arpa.2023-0187-ED.)

As Dr. Allison walks through the five best practices to help pathologists make that distinction, their workaday nature quickly becomes obvious.

No. 1: “Basically you want to use ASCO-CAP scoring criteria for IHC HER2,” which is what researchers used in the Destiny-B04 clinical trial. “There’s no new scoring criteria or algorithm.”

But since the criteria were not designed to qualify patients for antibody-drug conjugates, that brings up best practice No. 2: Pathologists should examine HER2 IHC at high power—40×—when trying to discriminate 0 versus 1+ staining. That’s now a clinically relevant, although not validated, threshold for whether to treat metastatic patients who are eligible for T-DXd. For now, Dr. Allison emphasizes, the focus is on patients with metastatic disease (though that could change). “But we want to be diligent for all of our scoring for all samples, including primaries, because that data may become relevant if the patient becomes metastatic.”

No. 3 suggests that for cases that present a scoring challenge, consider having a second pathologist review cases that are close to the interpretive threshold, i.e. greater than 10 percent of cells with incomplete membrane staining that is faint/barely perceptible.

Another trial, Destiny-B06, is looking more closely at those hard-to-distinguish cases, Dr. Allison notes. “They’re terming cases with some partial membrane staining but that falls just below the 10 percent threshold for 1+ ‘ultralow.’ But that’s for the future.” For now, she says, “We just do our best using the criteria we have.” Having a second pathologist is similar to recommendations that call for a second pathologist to review borderline ER cases. “Anytime you’re close to a threshold it’s a good idea to consider having someone else agree with you or disagree with you, so you know you’re all resulting similarly in your practice,” Dr. Allison says.

Best practice No. 4 calls for using controls with a wide range of protein expression, including 1+, to ensure an appropriate level of detection. Dr. Allison says on-slide controls are recommended, though not required.

At Stanford, she and her colleagues use a tissue microarray slide control, with multiple dots of tissue with different IHC expression including 0 and 1+. “It’s a little bit of a moving target,” she says, “because the clinical trial didn’t really test the threshold between 0 and 1+ so there are no clinically validated controls to calibrate to. But you do want to make sure your assay is not only able to pick up the highest levels of 3+ protein expression, so including cases that test 1+ in your lab will help ensure your assay is not becoming less sensitive.”

No. 5 guides labs to look at the preanalytic conditions to ensure that samples from both primary and metastatic sites have appropriate fixation and cold ischemic times. That can be a particular problem for metastatic sites, Dr. Allison cautions, since when a lung lesion, for example, or brain or liver metastases are biopsied, the pathologist may not know up front that the primary tumor is breast cancer. “If breast cancer is suspected in any metastatic site, pay attention to those preanalytic variables, since they can affect HER2 testing, and probably other tests as well,” she says.

How difficult will it be to put these best practices to work? “It depends on what’s within the pathologist’s control,” Dr. Jaffer says. “I can control the magnification I’m going to use to inspect the slide.” Preanalytical details are another matter. “Setting up standards for the multidisciplinary team is initially challenging. But with due course of time and action, it gets better.”

Using 40× to look at 0 and 1+ cases should be “fairly standard,” Dr. Allison says. “It’s just a matter of how hard you are looking”—like the difference between witnessing a bank robbery and tracking down embezzlement. “Before, we were looking for the high end of the spectrum, which was obvious. You could see it—it was very strong.” At the lower end, “you kind of hemmed and hawed,” she says.

Second pathologist reviews are fairly standard as well, though it doesn’t guarantee consensus.

Studies have shown that agreement is “pretty poor if you’re unaware there’s a difference between 0 and 1+,” Dr. Allison says. “But when you pay attention, and there’s more training, it’s higher.” Nevertheless, “Borderline cases are borderline cases,” she says, and are likely to have pathologist-to-pathologist disagreement.

So the point, Dr. Allison continues, is, “While we may not be sure what the most accurate result is, make sure you have concordance at least in your group.” Otherwise, the oboe has left the stage: “There’s nothing else we can fine-tune to.”

Ideally, Dr. Jaffer says, the second pathologist will have expertise in breast—“someone who does it day in, day out.”

Dr. Schnitt recalls discussing borderline cases with his colleagues during his work as a member of the pathology group for the European Society for Medical Oncology guideline. “Say you have six pathologists looking at the same case, and half would call it 0 and half would call it 1+. Do you err on the side of overcalling, or do you err on the side of undercalling?” he asks.

He suspects pathologists tend to overcall cases that are on the fence. “I talked to an oncologist from Italy several months ago, and he told me he’s basically not seeing any HER2 0s anymore,” he says with a laugh. “People are afraid they’re going to miss something that’s going to benefit the patient, and none of us wants to do that.”

In his own practice, Dr. Schnitt has turned to his oncologists for guidance. “I’ve specifically asked them, in borderline cases, would they rather have us undercall or overcall?” Most lean toward overcalling, he says, though “they’re very cagey about it. They basically say, We want you to be accurate; we want you to get it right. But the reality is, in these borderline cases it’s difficult.” And while medical oncologists are quite savvy, he says, “Some of them don’t understand that we’re using a test to make a distinction that wasn’t designed for that purpose.”

Using a wide range of controls might be the hardest of the best practices to, well, practice, Dr. Schnitt says. “The toughest thing may be to get controls that are 1+. Not that it’s terribly difficult, but people have to change the way they do controls.” Options include using cell lines that are embedded in cell blocks, he says, or using tissue sections embedded in paraffin that are 1+.

As for adhering to preanalytic conditions, Dr. Jaffer is a stickler. This is routine with primary tumors, she notes, but now, “if we’re getting a sample from a metastatic site, we need to let the oncologists and the person who’s procuring the tissue know that the same principles apply.”

She pursues a low-tech approach to convincing her clinical colleagues, using pictures from the literature to show the impact of delaying ischemic time. “When you show that to the surgeon, pictures are louder than words. They get it. And they start working with you to achieve it.”

The call for a footnote acknowledges the linguistic pachyderm in the room: As the authors note, “It is premature to change reporting terminology for lower levels of HER2 IHC expression,” even as scrutiny of these cases has increased. The goal was to avoid the confusion of introducing terminology that is likely to continue to evolve, and which hasn’t been validated.

Medical oncologists know they have new treatment options and will understand the implications of the various IHC scores, Dr. Allison says. They’re not necessarily looking for the words “HER2 low” and can get the information they need from the IHC raw score.

HER2 low may be a convenient and easy-to-say label, but it’s not a new subtype of breast cancer. “I think there’s a lot of misinformation about that—that there’s a new subtype, and we have to identify it,” Dr. Allison says.

But the term has become commonplace and, as noted, was used in Destiny-B04. Pulling it from use seems akin to pushing a barrel back up to the top of Niagara Falls. The irony is not lost on Dr. Allison. “The goal of the update was to end confusion,” she says with a laugh. “But the phrase is out there. And let’s be honest—it’s a great marketing tool, right? There’s a new subtype, and we need to find it.