Circulating tumour DNA (ctDNA)
Circulating tumour DNA (ctDNA) consists of fragments of DNA shed by tumour cells into the blood, detectable with sensitive sequencing.
Typically <1% of cell-free DNA. Uses: genotyping (companion diagnostics), resistance tracking (EGFR T790M, ESR1), MRD, early detection, response monitoring. Shedding varies by tumour type and burden; clonal haematopoiesis confounds.
After bladder removal, a blood test can now tell who needs immunotherapy and who can safely be spared it. This is the model for MRD-guided adjuvant therapy across cancers: treat the blood-positive, watch the blood-negative.
Patients newly diagnosed with EGFR-mutated advanced lung cancer now have a first-line option that improves survival over osimertinib, particularly if they have high-risk features. The trade-off is intravenous (now subcutaneous) infusions and considerably more skin, nail and clotting toxicity, so osimertinib alone remains reasonable for those who prioritise convenience and tolerability. Both this regimen and osimertinib plus chemotherapy (FLAURA2) are approved; there is no direct comparison.
The blood test stratifies risk far better than stage or pathology. It supports treating ctDNA-positive patients and suggests ctDNA-negative patients gain little from chemotherapy, but because treatment was not randomised the de-escalation claim needs the randomised trials that are now under way.
Relapse after surgery is driven by particular subclones that can be identified in the primary tumour and tracked in blood, which argues for evolution-aware adjuvant strategies. The pollution finding reframes carcinogenesis: some agents promote already-mutant cells rather than causing mutations.
For stage II colon cancer, where most patients are cured by surgery alone, a blood test can identify the minority who benefit from chemotherapy and spare everyone else its side effects. It does not yet prove that treating ctDNA-positive patients improves survival compared with not treating them.
A blood test can find early, treatable cancers in people who feel well, including cancers for which no screening exists. It is not a replacement for mammography or colonoscopy but a possible addition. Larger randomised trials are needed to show benefit outweighs harm.
Lung cancers keep evolving after they form, and it is ongoing chromosomal instability rather than the number of mutations that best predicts who will relapse. This gives a rationale for targeting the earliest (clonal) drivers and neoantigens and for tracking evolution in blood after surgery.
Many older people carry blood clones one or two steps from leukaemia, and those clones also drive heart disease through inflammation. CHIP is why blood-based cancer tests must filter out mutations from blood cells, and it opens a route to preventing both leukaemia and cardiovascular events in carriers.