Personalised blood tests which could track whether cancer treatment is working or if the disease has come back have been developed by US researchers.
The test identifies tumour DNA "rearrangements" which are specific to the individual patient.
In the future, this "genetic fingerprint" could be used to pick out tiny remnants of a tumour, Science Translational Medicine reports.
Such techniques are currently very expensive but costs are falling.
The researchers hope that one day the technology could be used to spot cancer recurrence before they would be picked up by scans.
DNA from volunteer patients was scanned for rearrangements of large chunks of genetic information which occur in cancer cells but not normal cells.
Known as personalised analysis of rearranged ends (Pare), the technique was developed using six sets of cancerous and normal tissue samples taken from four patients with bowel cancer and two with breast tumours.
They found between four and 15 DNA rearrangements in each of the six samples.
Using blood samples from two of the colorectal cancer patients, they found the test was sensitive enough to detect this marker or "fingerprint" DNA that had been shed by tumours into the bloodstream.
In tests on one patient, after surgery the levels of the marker DNA dropped due to the removal of the main tumour.
Then they rose again, suggesting that some cancer remained.
After chemotherapy and another round of surgery levels of the DNA markers fell once more.
The test was still picking up signs of the tumour which tallied with a small cancerous lesion in the patient's liver where the cancer had spread.
Cost
Further research is needed to ensure such a test could accurately detect cancer recurrence.
One current drawback is the expense with genetic sequencing costing about £3,200 per patient but, say researchers, costs are falling as the technology improves making the approach potentially more feasible.
Study leader Dr Victor Velculescu, from Johns Hopkins Kimmel Cancer Center in Baltimore, said: "I'm quite optimistic that within five years this approach could be turned into something that's widely applicable.
"Cancer more and more is becoming a chronic disease, and to manage a chronic disease you have to know how it's doing - is it getting better or worse?
"We haven't had any good ways of measuring how a cancer is doing up until now."
Co-author Dr Luis Diaz said: "Eventually we believe this type of approach could be used to detect recurrent cancers before they are found by conventional imaging methods like CT scans,"
Professor Peter Johnson, Cancer Research UK's chief clinician, said: "The detection of DNA changes, unique to individual cancers, has proved to be a powerful tool in guiding the treatment of leukaemia.
"If this can be done for other types of cancer like bowel, breast and prostate it will help us to bring new treatments to patients better and faster than ever."
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