HER2/neu Immunostaining in Invasive Breast Cancer: Analysis of False Positive Factors.

OBJECTIVES HER2/neu gene amplification by Fluorescent in situ hybridization and protein expression by immunohistochemistry have been used for prognosis and guidance for the treatment of invasive ductal carcinoma of the breast with Trastuzumab. False positive results are a significant problem where immunohistochemistry is exclusively used to test HER2/neu protein over expression. A minority of cases of breast cancer scoring HER2 (3+) by immunohistochemistry using Hercep test may not be associated with amplification of the HER2/neu gene by FISH, a test which is a more specific and sensitive than immunohistochemistry. This study aims to examine the factors contributing to false positive results by immunohistochemistry and subsequently not showing HER2/neu gene amplification by FISH analysis. METHODS A retrospective analysis of 18 cases (3+) by immunohistochemistry in the pathology laboratory not associated with HER2/neu gene amplification was performed. The histological review of these cases was done, the technical error (i.e staining of blood vessels or benign ducts) and the interpretation errors were evaluated. RESULTS Polysomy 17 was absent in all the cases studied by FISH analysis. By immunohistochemistry, five of the 18 cases were purely interpretation errors and the remaining were a combination of technical and interpretational errors. CONCLUSION False positive results related to technical and interpretational errors can be prevented by properly educating the technologist and pathologist to perform high quality immunostains and to render an accurate diagnosis respectively. This issue is of utmost importance as it may have deleterious effects on the selection of therapeutic arsenal in invasive ductal carcinoma of the breast.