To the layman stem cells seem to have miraculous powers. They can divide to produce copies of themselves and, taken from embryos that are just a few days old, can turn into any of 300 types of cell.

The bad news is that many cancer specialists now believe that cancerous growths have their own version of stem cells and that these cancer stem cells are responsible for starting a tumour and making it grow back after surgery or chemotherapy.

Professor Fiona Watt, of Cancer Research UK, says: “The idea of cancer stem cells has been around for decades. Now a critical mass of people in the cancer field accept their existence and are studying them in many different ways.

“If you look at a big tumour not all the cells in the mass are equally bad. Most of the cells are not capable of sustaining the tumour and just add bulk. Some are what are known as cancer stem cells, making up about 5 to 10 per cent of the tumour’s bulk. We now know that we have to target the stem cells that initiate the tumour and make it grow back. Identifying more cancer stem cells will lead to different therapies. It’s possible that some conventional treatments shrink the tumour but are not good at killing the stem cells.”

So far cancer stem cells have been identified in some breast cancers, as well as some cancers of the blood, brain, head, neck and intestines. If doctors can target the stem cells not only are the chances of effective treatment greater, but less damage will be done to the surrounding tissue.

New “targeted” therapies used with traditional chemotherapy have already been used to hit stem cells to get to the root of breast cancer and reduce the threat of relapses caused by regrowth.

Dr Michael Lewis, assistant professor of molecular and cellular biology, at the Baylor College of Medicine in Houston, said: “It’s not enough to kill the dandelion blossom and stalk. You have to kill the root as well. It appears that these cancer stem cells are resistant to the effects of anticancer drugs.”