In real-world practice, oncologists are granting treatment breaks, or 'holidays,' to metastatic bladder cancer patients who achieve major responses on enfortumab vedotin-pembrolizumab. This practice, driven by toxicity management and quality of life concerns, is common despite the lack of formal trial data to guide the optimal duration or timing of discontinuation.

Not all ctDNA clearance is equal. Data from KEYNOTE-361 shows chemotherapy clears ctDNA more frequently (40%) but with poor correlation to outcomes. In contrast, immunotherapy clears ctDNA less often (11%), but those patients who do clear it experience brilliant, more durable outcomes, suggesting a different biological mechanism of response.

The type of treatment inducing ctDNA clearance matters. Clearance from immunotherapy appears to be more permanent and strongly prognostic than clearance from chemotherapy, which can be transient. This suggests immunotherapy may achieve a more profound and lasting elimination of cancer cells versus cytotoxic agents.

While not yet validated, ctDNA is being used by clinical experts as a de-escalation tool to provide confidence when stopping long-term maintenance therapies like PARP inhibitors. This novel application focuses on reducing treatment burden rather than solely detecting disease progression.

An expert oncologist advises against ordering ctDNA tests that merely provide a "good or a bad feeling" about prognosis. The most valuable use is when a positive or negative result clearly dictates a clinical action, such as when to stop or restart adjuvant therapy.

Despite significant interest, circulating tumor DNA (ctDNA) is not yet an actionable tool for guiding the duration of maintenance immunotherapy in endometrial cancer. While studies like DuoE show ctDNA levels correlate with outcomes, there is no evidence to support using its clearance to decide when to stop treatment. It remains a prognostic, not a predictive, biomarker for this purpose.

In neoadjuvant therapy, a patient's long-term outcome is better predicted by stopping tumor DNA shedding (ctDNA clearance) than by achieving pathologic complete response (pCR), the traditional gold standard. This redefines what constitutes a successful treatment response before surgery.

After immunotherapy, many colorectal cancer patients have residual nodules on scans that appear to be partial responses. However, ctDNA testing can confirm these are often just scar tissue, not active disease. This provides the confidence to stop therapy at the two-year mark and avoid unnecessary surgeries for what are effectively complete responses.

For ctDNA-positive patients with high relapse risk, a clinical conflict arises. 'Purists' advocate for treatments with prospective trial data (single-agent immunotherapy), while 'pragmatists' argue for using the most effective therapy available (like EV pembro), treating ctDNA positivity as early metastatic disease, even without specific trial evidence in that setting.