ER, PgR, HER2 expression rates seen in Q-Probes

Anne Paxton

June 2020—With release of the latest Q-Probes study, titled “Expression Rates in Invasive Breast Carcinoma,” the CAP Quality Practices Committee fills a gap by providing data collected from a diverse set of 21 U.S. laboratories on the average frequency of various ER, PgR, and HER2 expression results.

Because the CAP Laboratory Accreditation Program requires that laboratories make annual comparisons of how their results line up with published benchmarks, the committee wished to find out if data from multiple laboratories would support the accreditation program checklist recommendations. The resulting Q-Probes study is part of an effort “to help laboratories make sure their results are in the right ballpark,” says study coauthor and former committee member Daniel David Mais, MD, director of surgical pathology, University of Texas Health Science Center, San Antonio.

“With ER, PgR, and HER2 testing, there’s tremendous concern at all times that your antibodies and your IHC system are working correctly. So we have multiple checks built into the system, and one of them is the guidelines published by the CAP and American Society of Clinical Oncology to annually compare frequencies of expression to benchmarks,” Dr. Mais says. Few data were available on what those benchmarks were, however. “So, here, we assessed the results people were getting in their real-life, day-to-day practice settings.”

The Q-Probes data from 687 breast carcinoma cases generally support accreditation program checklist recommendations (ANP.22970), which are as follows: The overall proportion of ER-negative breast cancers should not exceed 30 percent, somewhat lower in postmenopausal patients; the proportion of ER-negative cases is considerably lower in well-differentiated carcinomas (less than 10 percent); the proportion of PgR-negative cases is 10 percent to 15 percent higher than for ER-negative; for HER2 studies, the overall proportion of HER2-positive breast cancers is 10 to 25 percent; and well-differentiated tumors and lobular carcinomas are almost uniformly ER-positive.

(In the 2020 checklists edition, released June 4, reference to PgR studies was removed because PgR is now considered a prognostic marker rather than predictive.)

In the study, the overall ER-negative rate was 14.4 percent; for PgR it was 24.9 percent. Well-differentiated tumors (97.4 percent) and lobular carcinomas (98.7 percent) were almost uniformly ER-positive.

“The Q-Probes report provides another way to fine-tune the benchmarks that laboratories look at on a day-to-day basis,” says former Quality Practices Committee member and study coauthor Anthony J. Guidi, MD, chair of pathology at Newton-Wellesley Hospital in Massachusetts. “The checklist recommendations have a few specific criteria, but this study provides more benchmark data the CAP can use to potentially refine the checklist criteria. The diverse group of 21 labs that submitted information are all CAP accredited, so we know they use CAP criteria in setting up their assays. And when you can use multi-institutional data, that’s a better way to set up benchmark data than relying on the mostly single-institutional data that’s in the published literature.”

While a number of studies address positivity rates, they tend to be from single institutions, says committee chair and study coauthor Richard W. Brown, MD, medical director for system laboratory services, Memorial Hermann Health System, Houston. “The power of this study is that there was a broad range of laboratory size and institution type, so the study provides a robust data set that laboratories can use for comparisons.”

The study found fairly uniform expression rates among the participating laboratories, Dr. Mais says. “That would suggest to me that testing across laboratories is fairly reproducible and reliable. The accreditation program recommends that the overall portion of ER-negative breast cancer not exceed 30 percent. So if you find you’re falling outside of the recommendation, if more than 30 percent of your patients are ER-negative, that would be somewhat biologically improbable. It would suggest that maybe there is something wrong with either your test system or your interpretations.”

Included in this study is the aggregate data, he says. “Our look at the data is agnostic as to what antibody clone and test systems people are using in their particular lab.” The authors plan, in a separate project, to break down the data by antibody clone to see if anything can be gleaned from that analysis.

One slightly surprising finding of the study was the HER2 positivity rate of nine percent. “That’s lower than most published studies and obviously important because this data helps you compare to real-world test results,” Dr. Mais says. Similarly, a relatively high proportion of low-grade tumors tested as HER2-positive. “We found a 3.2 percent HER2-positive rate in grade one tumors in the study”—also a surprising percentage because grade one tumors are generally HER2-negative. “So potentially some participating institutions could be undergrading tumors or overinterpreting the HER2, or their HER2 system is staining them too darkly.”

Because HER2 expression is strongly associated with high-grade tumors, Dr. Brown says, “the finding of a well-differentiated carcinoma with HER2 expression, while not impossible, raises concern for an immunostain that is not providing an appropriate level of sensitivity.”

If laboratories are outliers and starting to see too many grade one cases be HER2 3+ positive, Dr. Guidi suggests, “That should spur them to look at their assays in more detail and make sure everything is correct in terms of total fixation time and cold ischemic time, and make sure their positive and negative controls are appropriate.” Some of the 3+ positive grade one tumors may reflect undergrading the core due to sampling issues, he notes. “Sometimes it’s just that you are looking at a relatively small amount of tumor on a core; however, on the excised specimen it might not be grade one anymore. You might see mitotically active areas in the tumor and you might bump it up to a grade two.”

But because such cases are a little unusual, his lab decided to verify rare HER2 3+ grade one tumors with a FISH test. “In fact we perform confirmatory HER2 FISH testing in grade one invasive ductal cancers and in grade one and grade two invasive lobular cancers with 3+ HER2 staining because the percentage of true HER2-positive tumors in this cohort should be very low.”