Relying solely on Next-Generation Sequencing (NGS) is insufficient for HER2 testing in biliary tract cancers. Data shows NGS misses up to 15% of patients with HER2 overexpression detected by immunohistochemistry (IHC). Performing both tests is essential to avoid denying patients effective targeted therapies.

Effective treatment of HER2-driven NSCLC requires more than just identifying mutations. HER2 is a multiplexed biomarker where both genetic mutations (TKD and non-TKD) and protein overexpression (via IHC) are independently actionable. Comprehensive testing is crucial to ensure patients are eligible for the full range of available targeted therapies, including TKIs and ADCs.

Upcoming data for the HER2-ADC Disitamab Vedotin will test if its efficacy is enriched in HER2-high patients. This trial spotlights a key field-wide dilemma: is specific enrichment necessary, or does a "bystander effect," as seen in breast cancer, mean even low HER2 expression is enough, complicating patient selection and the drug's positioning.

While Trastuzumab deruxtecan (TDXD) is effective in HER2-low breast cancer, there is no evidence that it benefits patients with HER2-low or HER2-intermediate (IHC 2+/FISH negative) gastric cancer. Its use should be strictly limited to truly HER2-positive cases in this disease.

When a colorectal tumor loses HER2 protein expression (IHC 0) but retains HER2 gene amplification via NGS, the decision to continue HER2-targeted therapy is guided by the amplification copy number. A low copy number argues against continuing the targeted regimen.

Unlike in breast cancer, where HER2 IHC 2+ requires FISH confirmation, in gynecologic cancers an IHC 2+ result is often considered directly actionable for prescribing HER2-targeted ADCs like T-DXD. This reflects a different, less stringent clinical standard for biomarker-guided therapy in this setting.

For certain therapies like Enhertu, eligibility is based on immunohistochemistry (IHC), not NGS. Labs must run HER2 IHC in parallel because NGS, as a population-based test, can miss intratumoral heterogeneity (small clusters of positive cells) that IHC can detect, thus identifying more eligible patients for targeted therapy.

A negative liquid biopsy (ctDNA) result for HER2 amplification does not prove a patient is HER2-negative. The test's sensitivity is limited by tumor fraction in the blood. While a positive ctDNA result is highly specific and trustworthy, a negative result is simply 'not detected' and requires a tissue biopsy to definitively determine HER2 status for treatment decisions.

Experts question if HER2 status truly predicts ADC efficacy in urothelial cancer. The benefit seen across low-expression levels suggests HER2's main role may be simply to target the chemo payload to cancer cells, rather than indicating a specific biological dependency.