The CURRENT the American Society of Clinical Oncologists (ASCO) and College of American Pathologists (CAP) clinical practice guideline, which was issued in 2018, does not provide a category for equivocal FISH results.
Cases with an average HER2 signals/tumor cell of >4.0 and <6.0 with the HER2/CEP17 ratio of <2.0 were previously diagnosed as ISH equivocal for HER2. The current recommendation for these cases requires that confirmatory IHC be performed using sections from the same tissue sample used for ISH. Slides from both IHC and ISH should be reviewed together to guide the selection of areas to score by ISH.
It is uncertain as to whether patients with an average HER2 signals/tumor cell of >4.0 and <6.0 with the HER2/CEP17 ratio of <2.0 benefit from targeted HER2 therapy in the absence of HER2 overexpression (IHC 3+).
