If research done on dogs' ability to sniff out cancer is taken further, we may find those sniffer dogs at airports studying to be medical sniffers, as well. You may have read this story recently. Dr Carolyn Willis, a dermatologist at Amersham Hospital in Britain, decided to test stories she'd heard about dogs' uncanny ability to sniff out cancer. The stories have been around for a while. Some scientists dismissed them as 'shaggy dog' stories. But not Willis. She was fascinated by the story of one dog that persistently sniffed at a mole on his owner's leg. He even scratched at it if she was wearing shorts. Finally, the owner had the mole removed. When the mole was checked it turned out to be very early melanoma. Melanoma is a commonly fatal form of skin cancer and often the hardest to detect in time. It starts usually as an ordinary-looking mole. By the time it turns into something that causes symptoms - usually bleeding, itching and irritation - it has often spread too far to be cured. So, the dog's curiosity saved his owner's life. This and other stories about dogs being able to sniff out not just skin cancer, but also internal cancers such as breast cancer, made Willis put dogs to the test. She explains her reasoning in a report in the British Medical Journal: 'Tumours produce volatile organic compounds, which are released into the atmosphere through, for example, breath and sweat. Some of these organic compounds are likely to have distinctive odours; even when present in minute quantities, they could be detectable by dogs, with their exceptional olfactory acuity.' She collected urine specimens from people known to have bladder cancer - reasoning that if cancer can be smelled, it should be detectable in urine - and trained five dogs to recognise the smell. The dogs were different breeds - one was a cocker spaniel, another a mongrel - and were trained by volunteers from Hearing Dogs for the Deaf. They were taught to lie down next to the sample with cancer in it. Early in the training, the dogs were given one cancer sample and the rest just water to smell. But as they got better at their training they were given all kinds of urine samples, including from women who were menstruating or from people with other bladder diseases, the idea being to make sure they weren't just interested in the smell of blood or the products of inflammation. At one stage, all the dogs worried their trainers by insisting one of these 'normal' samples smelled like cancer. When the person who provided the urine was examined, doctors discovered an undetected kidney cancer. Once the trainers were satisfied the dogs were getting it right, they were put to the test and given one sample of urine from cancer patients and six samples of 'normal' urine. The cancer patients were all new to the dogs, to make sure they weren't just recognising a scent of someone they had met before - the way dogs track each other's scent on urine patches and leave a bit of 'pee-mail' in reply. The odds that the dogs might just guess right were one in seven. As a group, the dogs got it right 22 out of 54 times - a success rate of 41 per cent. Some of the dogs - especially three young cocker spaniels - got it right more often; the least successful, a mongrel, only got it right once. However, the success rate dropped off as the tests went on, suggesting the dogs got tired or just confused by all the smells. Whatever the case, the medical world is now convinced that dogs can, indeed, smell cancer. But as Willis has said, the next step is not to put a dog in every waiting room, but to try to learn what molecules in the cancer are alerting the dogs so we can develop a mechanical test to detect it. But I think it's also great evidence that dogs really are our best friends. So, the next time a visitor is frightened by or cranky about your dog's sniffing habits, you've got a ready-made reply: he's just doing a free cancer check!