2006 -- By Elizabeth Cohen -- SAN ANSELMO, California -- Everyone loves their dog, but Nancy Best really loves her dog. That's because Nancy's sure her dog Mia saved her life.

Six years ago, Nancy was napping on her couch at home in Garberville, California, when Mia pounced and buried her nose in her right breast. Nancy thought nothing of it -- until over the next few days Mia did it three more times in the exact same spot.

The third time, Mia plunged with such force it hurt. Nancy reached to the spot Mia kept nosing at and felt a lump. She went to her doctor, and the biopsy came back positive: Stage II breast cancer.

Nancy underwent surgery and chemotherapy, and says she is alive today because of Mia.

This isn't the first time there've been reports of dogs smelling cancer. In 1989, an article in the respected British medical journal The Lancet reported that a dog kept sniffing a mole on his owner's leg, and the mole turned out to be melanoma.

Then in 2004, the prestigious British Medical Journal published a study where dogs managed to pick out urine samples from patients with bladder cancer set next to samples from healthy patients.

The 36 dogs didn't always pick out the cancer every time -- in fact, only 41 percent of the time -- but it was better than the 14 percent expected by chance alone.

What specifically did these dogs smell? Some scientists theorize there are certain "biomarkers" in cancer -- proteins, perhaps -- that might be unique to cancer and smelly to a dog's sensitive nose but undetectable to ours.

If this is true, it makes some sense that dogs could smell a tumor on the skin's surface (or, in Nancy Bests's case, near the surface) or in urine when the cancer was in the bladder.

But Michael Broffman and Michael McCulloch wanted to know if cancer could be smelled on a dog's breath. The two work at the Pine Street Foundation, an alternative medicine center outside San Francisco.

They took breath samples from patients with breast and lung cancer, and from people who were cancer-free. They took the cancer samples, and repeatedly over a three week period stuck them under the noses of five ordinary household dogs. When the dogs smelled cancer, they got a treat. When they smelled the healthy samples, they got nothing.

The researchers then arranged four healthy and one cancer sample in a line, and the dogs picked out the cancer with amazing accuracy: 88 percent with the breast cancer samples, and 99 percent with lung cancers.

When we visited with our cameras, one of the dogs, Kobi, re-created the study for us. Time and again, Kobi picked out the cancer sample, no matter where along the line they placed it.

So is the point to eventually have a dog in every doctor's office?

No. The researchers at the Pine Street Foundation are hoping someone will figure out what the dogs smell and then develop a test -- an electronic nose, if you will -- to detect it in breath, urine, or something else.

But there are skeptics. Dr. Donald Berry, head of biostatistics at the M.D. Anderson Cancer Center in Houston, suspects that the dogs' astounding accuracy has something to do with the way the test was done -- for example, the way the samples were collected.

"It may be true," he said, "but I would be astounded if it were true."

Another skeptic is Dr. Larry Myers, a renowned veterinarian and dog trainer at Auburn University in Alabama.

Myers said it takes him 13 weeks to train a dog to sniff bombs, and doubts anyone could train a dog to sniff cancer in just three weeks.