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.

Instead of just measuring the presence or quantity of proteins, new technology analyzes their physical proximity and co-localization on a cell's surface. This protein "geography" creates a unique spatial fingerprint that can more accurately distinguish healthy regenerating cells from residual cancer cells post-treatment.

The novel target DLL3 is a promising avenue for treating NECs, but its therapeutic efficacy is highly dependent on expression levels. Early trial data for a DLL3 T-cell engager showed a 40% response rate in high-expressing tumors versus just 3% in low-expressors, mandating biomarker testing for patient selection.

Despite widespread use in oncology, immunotherapy has limited efficacy in extrapulmonary NEC. Real-world data for combination CTLA-4 and PD-1 inhibitors show response rates around 15%, with sustained responses in less than 10% of patients. This highlights the urgent need for novel therapeutic approaches beyond checkpoint inhibition.

While Next-Gen Sequencing (NGS) provides genetic data, IHC directly measures the protein, is faster, cheaper, and requires less tissue. This makes it more scalable for routine clinical use, especially with small biopsy samples. High-level IHC loss correlates well with genetic loss seen on NGS.

Contrary to common belief, HER2 can be expressed or amplified in prostate cancer, particularly in subtypes with neuroendocrine features. This creates a rare but actionable target, with reported complete responses to HER2-directed therapies like TDXD, highlighting the need for broader genomic testing.

The molecular marker INI-1 loss, while characteristic of epithelioid sarcoma, is not pathognomonic. Other malignancies, like epithelioid angiosarcoma, can also have this finding. Clinicians must integrate molecular data with the patient's age and clinical presentation to confirm the diagnosis, sometimes requiring a re-biopsy in atypical cases.

A patient's clinical history can override ambiguous histology. A high-grade neuroendocrine neoplasm in a patient with a prior well-differentiated NET is almost certainly a progressive G3 NET, not a poorly differentiated NEC, as the two tumor types follow distinct biological progression pathways.

The primary site of a neuroendocrine neoplasm offers strong diagnostic clues. For example, tumors in the esophagus or gallbladder are almost always poorly-differentiated NECs, while those in the mid-colon are also typically NECs. This site-specificity serves as a powerful diagnostic heuristic for clinicians.