In neoadjuvant settings, ctDNA monitoring allows for real-time therapy adjustment. Data from the iSpy platform shows 80% of hormone-positive patients clear ctDNA with half the chemotherapy, enabling de-escalation, while the remaining 20% can be identified for escalated treatment.

A key conceptual shift is viewing ctDNA not as a statistical risk marker, but as direct detection of molecular residual disease (MRD). This framing, similar to how a CT scan identifies metastases, explains its high positive predictive value and justifies its use in making critical treatment decisions.

Dr. Pusztai clarifies the ctDNA lexicon: "Molecular relapse" is when a supposedly cured patient turns ctDNA positive during surveillance. "Molecular progression" is when a metastatic patient on therapy develops new resistance mutations detectable in ctDNA before clinical progression. This specific terminology is key for precise clinical decision-making.

The technology and breadth of molecular testing panels for GIST are rapidly evolving. A patient whose tumor tested negative for driver mutations in the past should be considered for re-testing with current, broader panels. This may uncover previously undetectable alterations like FGFR translocations and open up new treatment options.

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.

The InVigor11 study was the first to show that detecting recurrence via a ctDNA test before it's visible on scans is not just a prognostic sign, but an actionable clinical state. Intervening with therapy at this early stage was proven to improve patient outcomes, establishing a new paradigm for cancer surveillance.

Despite mutation testing being a critical first step for effective treatment planning in gastrointestinal stromal tumors (GIST), a significant number of patients in the United States still do not receive this essential diagnostic. This highlights a major gap between established best practices and real-world clinical application.

The presence of heterogeneous resistance mutations, some of which may be below detection limits, suggests a new strategy. Using a potent, broad-spectrum combination therapy upfront in the second-line setting, rather than sequential monotherapies, could eradicate more resistant clones and give patients a better chance at long-term survival or even a cure.

The interpretation of ctDNA is context-dependent. Unlike in the adjuvant setting, in the neoadjuvant setting, remaining ctDNA positive post-treatment signifies that the current therapy has failed. These high-risk patients need a different therapeutic approach, not an extension of the ineffective one.