HER2 testing in breast cancer: an overview of current techniques and recent developments

Testing for HER2 positivity in breast cancer carries implications for prognosis and therapeutic response in patients. In recent times there have been numerous developments and refinements in the available technologies for HER2 testing. In addition to this, guidelines have been developed and modified in an attempt to improve reliability and accuracy of testing. Immunohistochemistry and FISH testing have been the most widely used methodology, and the technique which has the largest knowledge base. Some of the inherent disadvantages have prompted the development of newer brightfield techniques which overcome some of these issues. There is gathering experience with these emerging technologies. Despite efforts to optimise and standardise procedures there remains a small percentage of cases that continue to be unresolved, whether this be due to issues of polysomy of chromosome 17, other complex genetic changes or analytical/interpretative issues. An ideal method for the resolution of these equivocal results should be considered in a specialised testing/referral centre, and this may include karyotyping studies of chromosome 17 or multiple probes for chromosome 17 using fluorescence in situ hybridisation or multiplex ligation-dependent probe amplification. It is timely to review of some of the newer techniques available for routine testing and approaches for cases which prove difficult to resolve using conventional testing methodology.