Watched another program on this topic today. Daily planet.
It gave a little more information on this. They have tested for lung and breast cancer with very high results of accuracy. Apparently the dogs are more accurate than the machines. They did a test on another cancer, sorry can't remember which one with the same high results.
They have tested stages 1-4 with the same high results so they think that they will be able to offer better treatment, earlier treatment.
They don't know what it is the dog is detecting but they are looking to find out. They have done variable tests to rule out confusion with other problems.