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.

ctDNA testing (liquid biopsy) is more effective than tissue biopsy for identifying ESR1 mutations. It samples DNA from all metastatic sites, capturing the disease's genetic heterogeneity and reflecting the most active resistance mechanisms, unlike a single-site needle biopsy which can miss them.

HER2 expression in cervical cancer can be heterogeneous and may emerge in metastatic sites even if the primary tumor was negative. Given the availability of effective HER2-targeting drugs, re-biopsying a metastatic focus is crucial to unlock previously unavailable treatment options for patients with recurrent disease.

Dr. Wander favors liquid biopsies for tracking disease progression because they are safer and easier for patients. While acknowledging that tissue biopsies can sometimes detect mutations missed by liquid ones (10-30% discordance), he believes rapidly advancing technology will soon minimize these discrepancies, making them the standard for monitoring.

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.

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.

When GIST progresses on therapy like imatinib, resistance is often heterogeneous. Different metastatic sites within the same patient can develop distinct secondary resistance mutations (e.g., an exon 13 mutation in the liver and an exon 17 in the peritoneum). This complicates subsequent treatment selection and underscores the value of comprehensive testing like ctDNA.

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.

Glioblastoma isn't a single mass but has finger-like 'tentacles' (diffuse infiltration) extending into brain tissue. It is also genetically and cellularly diverse, meaning a single-pathway drug will inevitably miss many tumor cells, leading to rapid recurrence and treatment failure.