They're known as man's best friend; but dogs could soon also be their greatest ally in the fight against prostate cancer. Britain's National Health Service recently approved a trial for dogs capable of sniffing out prostate cancer in the hope that it could show up inaccuracies in the current PSA (prostate specific antigen) test.

It's long been known that a dog's remarkable sense of smell can detect minute odors known to be associated with many cancers which are understood to be linked to volatile organic compounds produced by malignant cells.

"Dogs have got this fantastic sense of smell; three-hundred million sensory receptors, us humans have five million. So they're very, very good at finding minute odors. What we now know is that cancer cells that are dividing differently have different volatile organic compounds -- smelly compounds -- that are associated with the cells. And dogs with their incredible sense of small can find these in things like breath and urine," said Dr. Claire Guest who co-founded charity Medical Detection Dogs in 2008 to train specialist dogs to detect human diseases.

She added that the dogs' ability to sense chemical changes has been known throughout history but overlooked by modern medicine: "What dogs are doing is actually revisiting a way in which diagnosis has been done centuries ago. It was understood then that different volatiles - or smelly compounds - could be involved with changes in our body and may in fact enable someone to make an accurate diagnosis. But this has been very much forgotten. What the dogs are doing is finding the odors from bio-chemical changes in our body and this is opening a new way of diagnosing diseases and conditions in the future."

Medical Detection Dogs gained approval from Milton Keynes University Hospital for further trials, after initial testing showed trained dogs can detect prostate tumors in urine in 93 percent of cases. The charity says dogs undergo training for a period of about six months, after which they can reliably identify urine with traces of cancer cells in it. At the charity's facility the dogs do the rounds, sniffing a machine that holds eight urine samples. When they detect the sample that contains cancer cells, they either stop and sit down by it, bark or lick the bottle to indicate they can smell the cancer. Dogs are initially rewarded when they detect any urine scent, and then later only rewarded when they successfully identify cancer cells in urine samples.

Dr. Guest says dogs can detect the scent of cancer almost instantly, meaning they could potentially check many more samples than a human could possibly do.

"These dogs have the ability to screen hundreds of samples in a day; it's something they find very easy, they enjoy their work. To them it's a hunt game - they find the cancer," she said.

For Dr. Guest, it was this 'game' that potentially saved her life. In 2009 her labrador Daisy made her aware that she was suffering from the early stages of breast cancer when she began to nudge Dr. Guest's chest. Daisy, now 11-years-old, is one of the dogs taking part in the trials in Milton Keynes.

According to the charity, there are strong reasons for such a study. Prostate cancer is the most common cancer in men in the UK, and the second most common cause of cancer death.

The current PSA test involves analyzing a blood sample for a specific protein produced by the prostate gland; an elevated level of which could indicate prostate cancer and require a biopsy to be carried out. But the test has a high 'false positive' rate meaning many men may undergo the invasive procedure unnecessarily, and many general practitioners are reluctant to use it.

The scientists hope that dogs could provide a second line cancer screening service that demonstrated a low false positive rate and higher accuracy. And if dogs can be proved to be a reliable screening tool, a test could eventually be developed that is far superior to the PSA test.

While the current trials are focused on training dogs to accurately detect prostate cancer in urine samples, Rowena Fletcher from Milton Keynes University Hospital says dogs' unique skill could make them a valuable resource for doctors in detecting many more diseases.

"A lot of different diseases could carry a chemical signature, and then really the dogs could be used potentially to look at any other diseases which also had a chemical signature," she said.

While many dog-lovers may think it would be nice to have a dog in every doctor's surgery to screen for cancer, ultimately that is not practical. Instead the scientists hope their research will lead to the invention of an electronic nose that will mimic that of a dog's.

"Ultimately we hope to use the information that the dogs produce to actually develop an electronic nose. So eventually you could have a machine that sits on your consultant's desk, you'd put the urine sample in it and it would tell you if it was positive or negative. That would be the ultimate aim," added Fletcher.

However, a viable 'electronic nose' is still many years away, with no technology able to get close to the sniffing power of man's four-legged friend.

"The problem the electronic nose scientist has is that currently their sensitivity is well below the dog. A dog can find parts per trillion; we had an electronic nose working alongside the dogs recently and they were unable to find anything below parts per million," said Dr. Guest.