The TRACK-ER study shows ctDNA positivity isn't just a future risk predictor. Nearly half (44%) of patients with a newly positive ctDNA test were found to have metastatic disease on imaging. This suggests ctDNA often detects existing, micrometastatic disease that standard scans miss, challenging the distinction between early-stage surveillance and managing overt metastatic cancer.

While the need for prospective trials dominates the ctDNA discussion, a more fundamental obstacle is the lack of standardization between assay types (e.g., tumor-informed vs. agnostic). Without a common measurement approach, data from disparate trials cannot be pooled to create a universally accepted surrogate endpoint for regulatory approval.

Despite emerging trial data, clinicians are not yet ready to change therapy based on ctDNA positivity alone. Key concerns cited include the absence of a proven survival benefit from early intervention, the potential to use future treatment lines prematurely, and overall feasibility. The consensus is that while promising, the technology is not yet ready for routine clinical decision-making.

Advancing circulating tumor DNA (ctDNA) as a surrogate endpoint is stalled because the necessary large-scale, prospective validation studies are too expensive for any single company. The path forward requires a massive public-private partnership to fund research and establish standards, otherwise progress will remain incremental.

The original Signatera assay used 16 personalized probes based on whole-exome sequencing to find ctDNA. The next-generation version, based on whole-genome sequencing, expands this to 64 probes. This is expected to significantly increase sensitivity, detect molecular relapse earlier, and provide a longer window for clinical intervention.

Despite the promise of liquid biopsies for monitoring, the SERENA-6 trial revealed a significant challenge: fewer than 10% of screened patients developed a detectable ESR1 mutation. This low yield questions the efficiency and broad applicability of this serial screening strategy to guide treatment changes.

While promising, current ctDNA technology is not robust enough to justify stopping effective neoadjuvant systemic therapy in bladder cancer, even if a patient becomes ctDNA negative. Experts argue against using it to de-escalate treatment outside of a clinical trial due to the risk of undertreating a lethal disease.

Tumor-informed ctDNA assays, which require a tissue sample, are highly sensitive and well-suited for the adjuvant setting where tissue is available and time is less critical. In the metastatic setting, logistical challenges and the need for faster results make this approach less practical.

The main barrier to widespread ctDNA use is not its proven ability to predict who will recur (prognostic value). The challenge is the emerging, but not yet definitive, data on its ability to predict a patient's response to a specific therapy (predictive value).