San Francisco, CA—The preferred screening test for human epidermal growth factor receptor (HER)2 status in surgical esophageal adenocarcinoma specimens is immunohistochemistry (IHC), with fluorescence in situ hybridization (FISH) restricted to cases with an indeterminate (2+) IHC score, according to investigators from the Mayo Clinic, Rochester, MN, who proposed a testing algorithm at the 2013 Gastrointestinal Cancers Symposium.
“Our findings support a testing algorithm for resected esophageal adenocarcinomas where IHC is used for initial screening, and FISH testing is restricted to cases with equivocal IHC results,” said Harry H. Yoon, MD, Assistant Professor of Oncology at the Mayo Clinic College of Medicine and the lead investigator of the study.
HER2 oncoprotein overexpression is a known driver of tumor aggressiveness in esophageal adenocarcinoma; the HER2 gene is amplified in 7% to 22% of cases. HER2-positive patients treated with trastuzumab (Herceptin) have improved survival; therefore, it is important to test for HER2 with IHC or FISH, he said.
A previous exploratory, single-center study suggested that IHC is more predictive of trastuzumab benefit than FISH, but this superiority has not been established and each testing method has its advantages. IHC is faster, less labor intensive, and less expensive (as a result of the lower cost of reagents). FISH, on the other hand, is less vulnerable to tissue artifacts and offers a more objective scoring system, Dr Yoon explained.
The current study evaluated the concordance of HER2 test results between IHC and FISH assays in 673 patients with unselected surgically resected esophageal adenocarcinoma. Each tumor was evaluated by IHC in parallel with FISH in a blinded manner using US Food and Drug Administration–approved assays. A consensus IHC score was determined by 2 pathologists using tumor-specific criteria (negative, 0 or 1+; equivocal, 2+; positive, 3+). Gene amplification by FISH was defined as a HER2/CEP17 (chromosome 17) ratio of ≥2.
By FISH, 116 (17%) tumors tested positive. By IHC, 13% tested positive, 25% were indeterminate, 23% were negative by 1+, and 39% were negative by a 2+ score.
Among the 89 patients with an IHC 3+ score, 70 were FISH positive and 10 were FISH negative. Of the 107 indeterminate (2+) patients, 21 were FISH positive and 146 were FISH negative. Of the 417 IHC-negative patients, 16 were FISH positive and 401 were FISH negative.
This yielded a concordance rate of 95% between FISH and IHC 3+ patients and IHC-negative patients, and a 74% concordance rate when IHC 2+ patients were included.
HER2 amplification was detected in 89% of IHC 3+ cases, 13% of IHC 2+ cases, and in 4% of IHC 0 to 1+ cases (Table). Accordingly, using FISH as the reference standard, the positive predictive value of a positive IHC test (3+) was 89%, and the negative predictive value of a negative test was 96%. Importantly, the positive predictive value of an equivocal IHC score (2+) for detecting HER2 amplification was 13%, he said.
The sensitivity of IHC 2+ or 3+ was calculated to be 89%, and the specificity of IHC 3+ was 98%.
“In the largest study to date comparing HER2 testing methods in esophageal adenocarcinoma, a negative IHC result nearly excludes the presence of gene amplification by FISH,” Dr Yoon said, “whereas a positive result (3+) strongly predicts for the presence of amplification, and an equivocal IHC result (2+) is a weak predictor.”
