There's a growing recognition that the molecular profile of a primary tumor can differ significantly from its metastases. To guide treatment more accurately, the preferred practice is to biopsy an accessible metastatic lesion when possible, as this better reflects the biology of the active disease being treated.

An individual tumor can have hundreds of unique mutations, making it impossible to predict treatment response from a single genetic marker. This molecular chaos necessitates functional tests that measure a drug's actual effect on the patient's cells to determine the best therapy.

Contrary to concerns about over-complicating treatment, experts advocate for fragmenting gastric cancer even further. The goal is to treat each molecularly defined subset as its own distinct disease, which requires deeper understanding and more targeted approaches rather than broad simplification.

While re-biopsying at disease progression is the "by-the-book" standard to confirm biomarkers like HER2, clinicians acknowledge it is often skipped. The difficulty of obtaining tissue and the desire to provide patients with potential treatment options create a gap between guidelines and clinical reality.

Circulating tumor DNA (ctDNA) analysis allows for early detection of resistance mechanisms, such as secondary FGFR2 mutations, before tumors show growth on scans. This provides a potential window to adjust treatment strategies proactively, offering an advantage over traditional imaging-based monitoring.

Post-treatment ctDNA positivity is a powerful predictor of high recurrence risk in gastric cancer patients. However, this advanced diagnostic knowledge creates a clinical dilemma, as there is no evidence-based consensus on how to act on the results, forcing clinicians to make treatment decisions without supporting data.

Biomarkers provide value beyond predicting patient response. Their core function is to answer 'why' a treatment succeeded or failed. This explanatory power informs sequential therapy decisions and provides crucial scientific insights that advance the entire medical field, not just the individual patient's case.

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.

Clinicians advise against continuing targeted agents like zolbituximab or trastuzumab after disease progression in gastroesophageal cancer. The biological heterogeneity of this cancer type means that if a targeted therapy isn't working, it's unlikely to provide benefit with a different chemotherapy backbone.