Dogs can be trained to sniff out bladder cancer, the first controlled experiments published claim.
They are recognising a 'signature smell' of cancer
There have been anecdotal reports of dogs spotting cancer in their owners, but now researchers say they have proved this phenomenon scientifically.
The scientists at Amersham Hospital, Buckinghamshire, ultimately hope to build a tool that is as good at discerning these smells as dogs' noses.
Their findings appear in the British Medical Journal.
In 1989, researchers wrote a letter to the Lancet medical journal about how a woman claimed to have sought medical help as a direct result of her dog's inordinate interest in a skin lesion that turned out to be skin cancer.
Similar anecdotal claims have been made about cancers of internal organs like the breast and lung.
Cancers are thought to produce distinctive odours.
Even when present in minute quantities, it is possible that dogs, with their exceptional sense of smell, might be able to detect these odours.
Dr Carolyn Willis and colleagues conducted a carefully controlled experiment to see whether dogs could be trained to spot bladder cancer based on the odour of urine samples.
Over seven months, they trained six dogs of varying breeds and ages to discriminate between urine from patients with bladder cancer and urine from patients without bladder cancer.
On nine different occasions, each dog was offered a set of seven urine samples, of which only one came from a patient with bladder cancer.
Overall, the dogs correctly selected the bladder cancer urine on 22 out of 54 occasions.
This success rate of 41% was significantly more than the 14% that could be expected by chance alone.
Also, all of the dogs indicated one of the "bladder cancer free" samples as positive. This patient had been investigated prior to the study and no tumour had been found.
The patient's doctor was sufficiently concerned by the dogs' behaviour to do further tests.
These revealed a tumour in the patient's right kidney which had escaped diagnosis by usual medical tests.
Lead researcher Dr Willis said: "We are very excited because this is the first time this has been scientifically proven.
"Dogs have these fantastic olfactory abilities. .
"They are recognising a signature smell of cancer which is very difficult to pick up by any chemical methods.
"They are not just detecting a single chemical.
"They were having to pick out smells for bladder cancer amongst the hundreds in urine and that's no mean feat."
The dogs' trainer Claire Guest said it was a bit like naming the ingredients of a soup.
"We looked at a whole range of dogs. The spaniels did the best...but we are still keeping an open mind as to what breed of dog might be best for the job."
The researchers hope to be able to identify the exact cocktail of chemicals the dogs were smelling.
Then they might be able to design a medical device to detect these signature odours and pick up cancers in patients.
They will also investigate whether dogs can detect other cancers in a similar way, starting with skin cancer.
Cancer Research UK's Professor David Neal said: "Using sniffer dogs to detect the minute traces of molecules associated with cancer is a fascinating concept.
"Many cancer patients do have abnormal proteins in their blood and urine.
"The dogs might be smelling proteins from inflammation rather than the tumour itself, although the researchers have tried to minimise this possibility."
He questioned whether it would be practical to use dogs to detect cancers in real life, but said it might be possible to develop other detection methods based on future research in this area.