Molecular Residual Disease: Using ctDNA to Detect Cancer That Imaging Cannot

The Concept

Tumours shed fragmented DNA into the bloodstream. After treatment that aims to eliminate all cancer, a positive circulating tumour DNA test indicates residual disease that will, in most studies, eventually recur, often months before it becomes visible on imaging.

A negative test is more favourable but does not guarantee cure, because DNA shedding can fall below the assay's detection limit.

Tumour-Informed vs Tumour-Naive Assays

Tumour-informed assays first sequence the resected tumour, design a personalised panel targeting that patient's specific mutations, and then track those in blood. This gives very high specificity and sensitivity down to very low tumour fractions.

Tumour-naive assays look for a fixed set of common alterations or methylation patterns without needing the tumour. They are more convenient but generally less sensitive for low-level residual disease.

The Sensitivity Challenge

Residual disease may contribute only a handful of mutant DNA molecules per millilitre of plasma. Assays push sensitivity by tracking many mutations at once, using error-corrected sequencing, and exploiting phased variants that occur together on the same DNA fragment.

Even so, a single negative time point has limited predictive value, and serial testing over time is more informative.

Confounders

Clonal haematopoiesis contributes mutant DNA from blood-cell clones, especially in genes such as DNMT3A, TET2 and TP53, and can cause false-positive calls if not filtered by sequencing matched white blood cells.

Biological factors such as tumour type, location and proliferation rate strongly affect how much DNA is shed, so assay performance is not uniform across cancers.

Interpretation Notes

Molecular residual disease is prognostic in many settings, but whether acting on a positive result, for example by starting or intensifying adjuvant therapy, improves outcomes is still being tested in randomised trials.

A result should be read with the assay type, whether it is tumour-informed, and the timing relative to treatment.

Key Takeaways

• ·A positive post-treatment ctDNA test signals residual disease and predicts recurrence in most studies.
• ·Tumour-informed assays are more sensitive than tumour-naive ones for low-level disease.
• ·Serial testing is more informative than a single time point.
• ·Whether acting on a positive result improves survival is still under trial investigation.