Story highlights Dogs' noses have 300 million sensors, compared with a human's 5 million

Some dogs can detect cancer's unique odors called volatile organic compounds

Vital Signs is a monthly program bringing viewers health stories from around the world

Milton Keynes, United Kingdom (CNN) Lucy, a cross between a Labrador retriever and an Irish water spaniel, failed miserably at guide dog school. As she was curious and easily excitable, random scents distracted Lucy from her master's path, and it wasn't long before she was unceremoniously kicked out.

But her owners knew their smart dog held promise. They decided, if her nose was getting her into trouble (she was after all, bred to be a hunting dog), why not train her to sniff out something useful?

For the next seven years, Lucy learned to sniff out bladder, kidney and prostate cancer, and was even used in a study. Over the years, she has been able to detect cancer correctly more than 95% of the time. That's better than some lab tests used to diagnose cancer.

Now, Lucy is part of one of the largest clinical trials of canine cancer detection. A British organization, Medical Detection Dogs , has eight dogs sniff out 3,000 urine samples from National Health Service patients to see whether they can discern who has cancer and who doesn't.

Claire Guest is the CEO of Medical Detection Dogs. Her fox red Labrador, Daisy, caught her breast cancer six years ago when she was 45. "She kept staring at me and lunging into my chest. It led me to find a lump," Guest remembers.

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