FISH Still a Very Important Tool to Monitor Breast Cancer: Review of Recent Literature, Alternative Methods, and Proposed Techniques.

Objectives: Fluorescent in situ hybridization has been the definitive modality in testing for overexpression of the Human Epidermal Growth Factor Receptor 2 (HER2) for decades to guide the appropriate treatment for cancer patients. In more recent years innovation and new techniques have been developed to supplant or even replace FISH as a standard method for biomarker testing. Alternative testing methods such polymerase chain reaction (PCR), next-generation sequencing (NGS), and other in situ hybridization (ISH)-derived techniques such as chromogenic-ISH (CISH) have been shown in multiple publications to have high concordance with FISH in addition to advantages in economics, logistics and practicality to the point where CISH and derived methods appear to have eclipsed FISH as a testing method of choice after immunohistochemistry (IHC). This review assesses the status of FISH compared to other diagnostic techniques such as IHC, CISH, and less common and/or experimental methods. Also addressed are the updates to the guidelines from the American Society of Clinical Oncology (ASCO), College of American Pathologists (CAP), and National Comprehensive Cancer Network (NCCN) regarding FISH and IHC for HER2 testing with the updates reducing the number of equivocal diagnoses in the latest iteration. Though our findings show a constantly changing technological landscape, FISH remains an important primary tool to guide medical treatment and as a solid foundation to build upon for innovation in cancer research.