But can they sniff out cancer?
The idea that dogs can detect cancer has been around for a while, perpetuated on the internet (for example here) and in a BBC4 documentary first broadcast in 2006.
There are many anecdotes of family pets persistently sniffing or worrying at a certain area on their owner, which subsequently turns out to harbour a tumour.
As a result, we receive numerous enquiries asking whether Cancer Research UK is investigating this phenomenon.
But is there any scientific basis behind the idea? And if there is, are dogs really a practical solution for cancer screening?
Picking up the scent
All smells – from the pleasant aroma of freshly cut grass to the whiff of Stinking Bishop – are due to molecules diffusing in the air, given off by whatever is causing the pong.
These so-called volatile molecules are detected by scent (olfactory) receptors in the noses of humans and animals, sending signals to the brain which then interprets the smell. Dogs have an astounding sense of smell, because their noses are packed with many times more scent receptors than humans have.
We know that some tumours produce unusual volatile molecules (such as lung cancer, breast cancer, and melanoma) which are presumably being picked up by the dogs in the stories mentioned above. But does this mean that we should have “cancer sniffer dogs” in every GP surgery?
Putting it to the test
A small number of studies have set out to directly investigate whether dogs can detect cancer under controlled conditions, and one of the key pieces of research is a paper by Carolyn Willis and her colleagues in the UK.
Her team trained six dogs to tell the difference between urine samples from bladder cancer patients and samples from healthy people. When the dogs were presented with new samples – six from healthy people and one from a cancer patient – the scientists found they could pick out urine samples from people with cancer 41 per cent of the time (22 out of 54 tests). If they were correct only by chance, we would expect the dogs to pick the right sample only 14 per cent of the time (i.e. one in seven).
Michael McCulloch in the US has also looked into the phenomenon. His team trained 5 ordinary dogs using breath samples from lung and breast cancer patients, as well as 83 samples from healthy people. Then they tested the dogs with new samples from patients or healthy people that they hadn’t previously sniffed.
The test was double-blinded, meaning that neither the dog handlers nor the researchers watching them knew which samples were which. The results were promising – with only a few weeks training, the team found that the dogs could correctly spot the samples from cancer patients with high levels of accuracy.
But another study by Robert Gordon and his colleagues in California showed less impressive results. In a study involving six dogs, only two performed better at detecting urine samples from patients with breast cancer than you might expect by chance. And in a test of four dogs, only two could pick out urine samples from prostate cancer patients at a rate better than predicted by chance.
It’s certainly a nice idea – man’s best friend sniffing out tumours. But although these small research studies show that dogs could have the potential to pick up cancer, their accuracy is questionable.
This is problematic because in the case of tests for cancer, it’s important to have a test that is as reliable and accurate as possible – although, of course, no test is ever 100 per cent infallible. This cuts down on the chance that cancers will be missed (false negatives), or that a diagnosis of cancer will be wrongly made (false positives).
Also, it’s simply not realistic to transfer this to a clinical setting. For a start, there are the practical considerations. Dogs would need to be trained, housed and fed, and would not be able to work for more than a few hours at a time for the sake of their welfare.
More importantly, it’s still not clear from the research that the dogs are picking up specific molecules originating from a tumour, or are just picking up molecules associated with more general illness. And there’s also the chance that they may be confused by other scent molecules on the breath or in urine, such as garlic, asparagus, tobacco, alcohol or other pungent foods.
Dogs are also – obviously – unable to tell doctors exactly which molecules are faulty in an individual’s cancer – information that can be obtained using molecular diagnostic techniques. This is important because such information is increasingly being used to tailor treatment to an individual’s cancer.
But although Fido won’t be wearing a canine labcoat any time soon, there’s a lot of research going on into detecting the scent of cancer.
Introducing the electronic nose
Although they don’t have furry coats, there are “electronic noses” that can pick up the volatile molecules produced by cancer cells with greater accuracy than a real life cold, wet one. This kind of technology is being tested in a range of cancers, including breast and lung.
It’s still early days for electronic noses, and we need to see a lot more research to make sure the machines are accurate and reliable at detecting cancer. But once the technology has advanced further, it’s likely that they will have wide applications for detecting many different types of cancer. And they won’t need feeding or walking either.
C DENG (2004). Investigation of volatile biomarkers in lung cancer blood using solid-phase microextraction and capillary gas chromatography?mass spectrometry Journal of Chromatography B, 808 (2), 269-277 DOI: 10.1016/j.jchromb.2004.05.015
Michael Phillips et al (2006). Prediction of breast cancer using volatile biomarkers in the breath Breast Cancer Research and Treatment, 99 (1), 19-21 DOI: 10.1007/s10549-006-9176-1
A. D’Amico et al (2008). Identification of melanoma with a gas sensor array Skin Research and Technology, 14 (2), 226-236 DOI: 10.1111/j.1600-0846.2007.00284.x
C. M Willis (2004). Olfactory detection of human bladder cancer by dogs: proof of principle study BMJ, 329 (7468) DOI: 10.1136/bmj.329.7468.712
M. McCulloch (2006). Diagnostic Accuracy of Canine Scent Detection in Early- and Late-Stage Lung and Breast Cancers Integrative Cancer Therapies, 5 (1), 30-39 DOI: 10.1177/1534735405285096
Robert T. Gordon et al (2008). The Use of Canines in the Detection of Human Cancers The Journal of Alternative and Complementary Medicine, 14 (1), 61-67 DOI: 10.1089/acm.2006.6408
K GENDRON et al (2007). In vitro discrimination of tumor cell lines with an electronic nose Otolaryngology – Head and Neck Surgery, 137 (2), 269-273 DOI: 10.1016/j.otohns.2007.02.005
H CHAN, C LEWIS, P THOMAS (2009). Exhaled breath analysis: Novel approach for early detection of lung cancer Lung Cancer, 63 (2), 164-168 DOI: 10.1016/j.lungcan.2008.05.020