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.

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.

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.

While liquid biopsies (ctDNA) excel at detecting mutations, tissue biopsies are irreplaceable for assessing the fundamental biology of the most life-threatening metastatic sites. For instance, a direct liver biopsy is needed to confirm estrogen receptor expression, a critical factor that ctDNA cannot determine.

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.

Before starting a second-line HER2-targeted therapy like trastuzumab deruxtecan, it is critical to re-biopsy the tumor if feasible. HER2 status can change after first-line treatment, and confirming persistent HER2 positivity is essential to ensure the subsequent therapy will be effective.

Patients with HER2-positive GI cancers can lose expression after treatment. While re-biopsy is ideal, it's often impractical or risky. In these cases, clinicians find ctDNA analysis of HER2 copy numbers to be a reliable alternative for guiding subsequent treatment decisions.

Unlike in other cancers, about one-third of HER2-positive gastroesophageal tumors become HER2-negative after first-line treatment. This loss is a key mechanism of resistance, making it mandatory to re-biopsy and re-test for HER2 at the time of disease progression to determine appropriate subsequent therapies.

Due to selective pressure from first-line treatment, 30-40% of HER2-positive gastroesophageal cancers lose HER2 expression by the time of progression. It is crucial to re-test these patients, either via tissue biopsy or ctDNA, to confirm continued HER2 positivity before initiating second-line HER2-targeted therapy like TDXD.