I actually work with these tests and it's the new big thing in cancer detection. Cell-free DNA detected in the blood plasma can be collected. When cells die off they release DNA into the bloodstream, having a half life of around only 30 minutes. Very, very short. If you have a small tumour, well-connected to the blood stream, the cells will die at a higher rate. That higher rate of cell death, via apoptosis or necrosis will increase the levels of cell-free DNA in your blood plasma. That's the first sign. Then if you create a targetted panel of genes to cancer, you can match up your cell-free DNA to detect mutations being given off.This method, should it continue to prove good, will help cancer screening. Many tumours can't be imaged until they are big enough, or a biopsy may be too difficult or dangerous to take.