The UK team was able to scour the blood for signs of cancer while it was just a tiny cluster of cells invisible to X-ray or CT scans.
It should allow doctors to hit the tumour earlier and increase the chances of a cure.
They also have new ideas for drugs after finding how unstable DNA fuels rampant cancer development.
The research project was on lung cancer, but the processes studied are so fundamental that they should apply across all cancer types.