IHC 2+ in biliary tract cancer: When to reflex to FISH—and what it means clinically
Industry Buzz
The key is to separate HER2 overexpression from HER2 amplification. If the pathology report says IHC 3+, treat this as HER2-positive overexpression. If the report says IHC 2+, this is an equivocal result and should prompt reflex ISH/FISH.
—Laura Vater, MD, MPH
HER2 alterations are present in 5%-20% of patients with cholangiocarcinoma and 15%-30% of those with gallbladder cancers, making this a clinically significant biomarker. While HER2 positivity is often associated with more aggressive disease and worse prognosis, it also opens the door to targeted therapies that can improve outcomes, particularly in the second-line setting.
Where diagnostic decisions impact treatment pathways
For patients with unresectable or metastatic bile duct cancer (BTC) who are candidates for systemic therapy, molecular profiling is essential at diagnosis to identify actionable targets and guide therapy choices.
Performing profiling upfront reduces the time needed to make second-line treatment decisions. It also helps in enrolling patients in biomarker-driven trials and offers information on prognosis and resistance mechanisms.
However, this is where things often start to break down in practice: Even when HER2 is present, the way you test (and interpret the results) can determine whether a patient ever reaches targeted therapy.
Which assays to use
Next generation sequencing (NGS) is the preferred method for comprehensive molecular profiling in BTC and should complement HER2 immunohistochemistry (IHC). Reflex FISH/ISH testing should also be performed if IHC results are 2+.
A practical approach is to request two things on the diagnostic or most recent biopsy sample:
A comprehensive DNA-based NGS panel that reports ERBB2 amplification and other actionable alterations
A dedicated HER2 IHC assay performed on tumor tissue.
Because concordance between amplification and protein overexpression is imperfect, relying on NGS alone can miss patients with clinically actionable HER2 overexpression. This is the main reason NGS plus IHC is favored in BTC.
The IHC-first, reflex-ISH approach originates from established gastroesophageal HER2 testing guidelines and is often applied to BTC, as BTC-specific scoring systems are less standardized. In this context, IHC 3+ is considered positive, IHC 0/1+ is negative, and IHC 2+ is equivocal and should prompt ISH/FISH testing.
How to interpret the report
The key is to separate HER2 overexpression from HER2 amplification. If the pathology report says IHC 3+, treat this as HER2-positive overexpression. If the report says IHC 2+, this is an equivocal result and should prompt reflex ISH/FISH.
An IHC 2+/ISH+ result is clinically meaningful but, by itself, may not meet eligibility criteria for targeted therapy in the US. It should prompt review for trial eligibility, discussion of off-label strategies when appropriate, and confirmation that no higher-quality IHC 3+ tissue block is available.
If the NGS report shows ERBB2 amplification, this indicates that the tumor may be HER2-driven, but amplification alone is not equivalent to documented IHC 3+ overexpression. In practice, an NGS report showing ERBB2 amplification should trigger review of the HER2 IHC result, and if IHC was not sent, it should be ordered.
If the report shows IHC 0 or 1+ and no ERBB2 amplification, current HER2-directed BTC treatment options are unlikely to be applicable outside a trial.
When it comes to HER2 testing in BTC, gaps in testing strategy translate into missed opportunities. Consistent use of both NGS and HER2 IHC, paired with clear interpretation pathways for equivocal or discordant results, supports timely identification of patients who may benefit from targeted therapy or clinical trials.
In a disease where treatment options remain limited, getting HER2 testing right—the first time—is one of the clearest opportunities to translate molecular insight into clinical benefit.
Related: Practice-changing signals amid evidence gaps: What’s driving your 2L decisions in HER2+ BTC?