Per NCCN and ASCO guidelines, liquid biopsy (ctDNA) is now preferred over tissue biopsy for detecting ESR1 mutations. Its advantages include being less invasive with a faster turnaround, but most importantly, it effectively captures the dynamic, evolving nature of tumors, which is characteristic of acquired ESR1 mutations.

Circulating tumor DNA (ctDNA) assays show high concordance with tissue biopsies and may yield a higher rate of identifying ESR1 mutations. This is because ctDNA captures tumor heterogeneity from multiple metastatic sites, which a single tissue sample can miss, providing a more comprehensive genomic picture.

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.

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.

Retesting for biomarkers with liquid biopsy in the third-line setting is crucial. It can uncover new, actionable mutations that have emerged during treatment or confirm the absence of resistance mutations, potentially allowing patients to benefit from re-challenging with a previously used targeted therapy.

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.

Liquid biopsies are superior for ESR1 testing because the mutation is subclonal and develops under therapy pressure. A tissue biopsy from one metastatic site can miss the mutation, while circulating tumor DNA (ctDNA) from a blood sample aggregates genetic material from all sites, increasing detection rates.

Despite the risk of missing mutations, oncologists predominantly use convenient, less-invasive liquid biopsies to test for biomarkers at disease progression. A more invasive tissue biopsy is generally reserved for situations where the cancer behaves unexpectedly, such as a sudden shift from bone-only to visceral disease, which might suggest a missed biological driver.

For patients with bone-predominant metastatic disease, repeating a tissue biopsy is often not feasible. A liquid biopsy (ctDNA analysis) is the preferred method as it is easier to perform and more likely to detect an ESR1 mutation compared to tissue-based NGS testing in this specific clinical scenario.