Of all animals, so far, the evidence shows that dogs are the best to detect early stage cancer in humans. Canine cancer detection is an approach to cancer screening that relies upon the olfactory ability of dogs to detect, in urine or in breath, very low concentrations of the alkanes and aromatic compounds generated by tumors. At the ACR Institute, we are working in getting at least one dog or cat that can be trained in this skill. For the details, see Source.
Among others, see the Institute’s workshops for many more early detection diagnostic tests.