Two previous studies have shown that dogs seem to be able to sniff out melanomas and bladder cancer.
[R]esearchers at the Pine Street Foundation in San Anselmo, California, US, selected three Labrador retrievers and two Portuguese water dogs with no previous training, and over several weeks trained them using breath samples that had been exhaled into tubes by cancer patients.
To test how well the dogs had learned, they used a new batch of samples and had the dogs attempt to distinguish among 55 lung cancer patients, 31 breast cancer patients and 83 healthy controls. The patients had all had their cancers confirmed by biopsy. The tests were double-blind, so neither the dog handlers nor the experimenters knew which tubes were which. . . The dogs correctly detected 99% of the lung cancer samples, and made a mistake with only 1% of the healthy controls. With breast cancer, they correctly detected 88% of the positive samples, and made a mistake on only 2% of the controls.
It isn't clear whether the dogs were specifically detecting cancer, or simply were able to the distinguish the breath of a healthy person from someone who was not healthy. But, it is undeniable that they detected something with a high diagnostic accuracy, presumably, with their acute sense of smell.