This excellent review by Drs. Fornier, Risio, Van Poznak, and Seidman provides an explanation of the regulatory pathways of cell proliferation that involve the epidermal growth factor receptor (EGFR) family. Ultimately, the authors focus on the differences in HER2 testing methods and, most importantly, the correlation between immunohistochemistry, fluorescence in situ hybridization (FISH), and clinical response. Unfortunately, in most of the reported studies, the concordance between immunohistochemistry and FISH may not reflect the real world of HER2 testing, because they use the results of both immunohistochemistry and FISH testing performed in experienced reference laboratories.
The need for accurate determination of HER2 status is becoming more apparent, as therapeutic decisions are based mainly on testing in the advanced setting, and now on clinical trials evaluating trastuzumab(Drug information on trastuzumab) (Herceptin) in combination with polychemotherapy in the adjuvant setting. Which test should be used to determine eligibility for therapy? Of greater concern and controversy, however, is the level of concordance between local and central laboratories.
Concordance Between Assays
Consistently, multiple reports have demonstrated high concordance between FISH and immunohistochemistry for cases scored as 0 and 3, especially when the testing was performed in highly experienced laboratories. In contrast, low concordance has been found for immunohistochemistry scores of 1+ or 2+.
One such important concordance study was presented by Mass and colleagues, who detected gene amplification in 3%, 7%, 24%, and 89% of samples with 0, 1+, 2+, 3+ HER2 overexpression by immunohistochemistry, respectively. Of note, this study had an artificially selected equal number of patients scored as negative (0 or 1+) and positive (2+ or 3+) by immunohistochemistry. This arbitrary division does not reflect the actual proportion of patients with these immunohistochemistry levels in the general breast cancer population. Tubbs et al have reached the same conclusions and have determined that the largest source of discordance involves HercepTest scores of 2+, again with a high rate of false-positive results.
A large cohort of patients analyzed by Perez and collaborators demonstrated that among patients with HER2 overexpression scores of 2+, only a minority had high levels of gene amplification. This information should be seriously considered, because both of the large single-agent trastuzumab trials included patients with 2+ or 3+ HER2 overexpression by immunohistochemistry. As will be discussed later, retrospective analysis showed that only 1 of 65 immunohistochemistry-positive but FISH-negative patients derived clinical benefit from trastuzumab therapy.
Another important concordance study of 117 breast cancer specimens was recently reported. Press et al found that the accuracy of the FISH assays was high97.4% for the Vysis PathVision and 95.7% for the Ventana INFORM test. The immunohistochemistry assay with the highest accuracy was the R60 polyclonal antibody with 96.6% and 95.7% for the 10H8 monoclonal immunohistochemistry antibody. Interestingly, the lowest accuracy was for the two commercially available immunohistochemistry tests, the Dako Hercep Test (88.9%) and the Ventana C11 monoclonal antibody (89.7%).