British researchers, who developed the new blood test, said can closely monitor the progress of tumours without the need for surgery or scans.
In a world first, they used the blood analysis to detect deadly mutations in 20 of 38 patients studied with breast and ovarian cancer. They even managed to build up a picture of how one woman's breast cancer responded to different treatments over 16 months.
The new test, expected to be available to patients within five years, uses sequencing -- a technique to read genetic code -- to look for tiny pieces of DNA that cancer tumours shed into the bloodstream and are not picked up by current screening methods, the Daily Mail reported.
This could alert doctors when a treatment is not working, allowing them to try alternatives, as well as potentially sparing the patient unnecessary side effects. Currently cancer patients often need to have tissue samples surgically removed for analysis.
In the study, published in journal Science Translational Medicine, the researchers took blood samples from women either before, during or after chemotherapy at Addenbrooke's Hospital in Cambridge.
Dr Nitzan Rosenfeld, who led the study at the Cancer Research UK Cambridge Research Institute, said: "This type of blood test has the potential to revolutionise the way we diagnose and treat cancer. The great advantage is that it can be used to identify cancer mutations without surgery or a biopsy, making it safer and cheaper."
DNA sequencing is widely used, but Rosenfeld's team found a way to home in on key genes known to be "cancer hot spots" and analyse them in detail.
This is the first time entire genes have been analysed from a blood test, which enabled the researchers to examine 20,000 possible mutations in six cancer-related genes. (More)