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.

While liquid biopsies are a valuable, less invasive tool, a negative result is inconclusive for ruling out actionable mutations in NSCLC. It may simply mean the tumor isn't shedding enough DNA. Therefore, a negative liquid biopsy should never be the final word; it must be followed by a tissue biopsy to ensure patients don't miss out on targeted therapies.

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.

Clinicians must recognize that liquid and solid biopsies show significant discordance. ESR1 mutations are more frequently detected in liquid assays, while PIK3CA mutations are more often found in solid tissue. This variability by gene directly impacts the optimal testing strategy for patients.

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.

When the FDA approves a new biomarker-linked therapy, an in-house pathology lab actively queries its historical database of all prior NGS tests to identify past cases with the relevant genetic alteration. They then proactively contact the oncologists for these patients, uncovering new treatment options that were previously unavailable.

When an oncologist anticipates an initial sample (e.g., cytology) will likely have insufficient tissue for NGS testing, they proactively initiate a biopsy of a second site with interventional radiology. This parallel-path approach avoids waiting for the first test to fail, significantly reducing time to diagnosis and treatment.

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.

A common clinical practice—biopsying the primary tumor to guide treatment for metastatic disease—is considered biologically flawed. Metastases can have vastly different molecular and immune profiles from the primary tumor and from each other. Experts advocate for re-biopsying metastatic sites when feasible to get a more accurate profile of the progressing disease.