The current American Society of Clinical Oncologists (ASCO) and College of American Pathologists (CAP) clinical practice guidelines, issued in 2018, do 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 confirming IHC using sections from the same tissue sample used for ISH. Slides from 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+).
