Guideline revisions lead to increase in HER-2–positive breast cancers, possible false positives

Changes to HER-2 testing guidelines made by ASCO and the American College of Pathologists in 2013 have significantly increased the number of patients with breast cancer who tested positive for HER-2, according to data published in Journal of Clinical Oncology.

However, this increase could represent an excess of false-positive cases, which could lead to inappropriate use of HER-2–directed therapies like trastuzumab (Herceptin, Genentech), according to the researchers.

“The new guidelines were established to reduce the number of equivocal cases, where HER-2 status is uncertain, but we found that they did just the opposite,” Robert B. Jenkins, MD, PhD, Ting Tsung and Wei Fong Chao professor of individualized medicine research and professor of laboratory medicine and pathology at Mayo Clinic in Rochester, Minnesota, said in a press release. “The number of equivocal cases went up, resulting in additional testing and a much larger number of women with cancers ultimately labeled as HER-2 positive.”

The FDA approved the use of HER-2 testing guidelines to determine the appropriateness of HER-2–directed therapies in 1998. ASCO and the American College of Pathologists initially published immunohistochemistry and fluorescent in situ hybridization (FISH) guidelines in 2007. The 2013 update changed the cutoff for equivocal and positive cases, defining HER-2 positivity in patients with an HER2 signal greater than or equal to 6 with a FISH ratio less than 2.

Jenkins and colleagues sought to determine the degree to which guideline changes influenced the identification of HER-2–positive breast cancers, working from a hypothesis that the 2013 update would lead to an increase in breast cancers testing positive.

The Mayo Clinic Cytogenetics Laboratory implemented the 2013 guideline update in November 2013. The researchers had access to 2,851 samples sent to the laboratory for FISH testing between November 2013 and October 2014.

Immunohistochemistry (IHC) results were available for 67.4% (n = 1,922) of patient samples (IHC 0, 2.4%; IHC 1+, 7.9%; IHC 2+, 84.8%; IHC 3+, 2.5%).

Based on 2013 guidelines, 1,986 (69.7%) of the patients were deemed HER-2 negative, 460 (16.1%) were deemed HER-2 positive and 405 (14.2%) were deemed equivocal.

Of patients deemed HER-2 positive, 339 (11.8%) had a FISH ratio of at least 2 and a HER2 signal of at least 4, whereas 35 (1.3%) had a FISH ratio of at least 2, despite a HER2 signal less than 4. Eighty-six patients (3%) were HER-2 positive with a HER2 signal of at least 6, despite a FISH ratio less than 2.

The 2007 guidelines would have interpreted 2,425 patients (85.1%) as HER-2 negative, 314 (11%) as HER-2 positive and 112 (3.9%) as equivocal.

The original FDA guidelines — which did not include an equivocal category — would have interpreted 374 patients (13.1%) as HER-2 positive and 2,477 patients (86.9%) as HER-2 negative.

Because the 2013 guidelines recommend alternative chromosome 17 probe testing to resolve equivocal results, researchers retested the 405 patients with initial equivocal results using the Mayo Clinic Cytogenetics Laboratory HER2/D17S122 probe set. These results reassigned 212 of 405 equivocal patients (7.4% of the overall cohort) to HER-2–positive status and 36 patients (1.3% of the overall cohort) to HER-2–negative status. The other 157 patients remained equivocal.

Thus, the final results using 2013 guidelines determined 24.1% of patients were HER-2 positive, which represented a significant increase compared with the 9.4% of patients deemed positive using the 2007 guidelines and 11.8% deemed positive using the initial FDA criteria (P < .001 for both).

The researchers acknowledged study limitations, including the use of a single reference laboratory and the potential for variability in initial IHC or FISH testing.

Overall, the 2013 guidelines increase the proportion of woman who would be eligible for HER-2–directed therapies, despite that these women would not have been eligible for the registration trials of these agents, according to the researchers.

“Women who receive false-positive results are not only exposed to the risks of HER-2–directed therapies, but they also miss out on the treatments that could be effective against their cancer,” Jenkins said. “That is counter to the goal of personalized medicine, which is to give the right drug to the right patient at the right time. Given the medical, financial and psychosocial aspects of these targeted therapies, it is prudent that we prospectively identify the most optimal candidates for treatment.” – by Cameron Kelsall

Disclosure: Jenkins reports royalties from Abbott Molecular and Genome Diagnostics. Other researchers report stock ownership in Merck and consultant roles with ARIAD Pharmaceuticals and Hospira.