The smell of cancer: from its detection by dogs to that by machines

Several observations indicate that dogs have the ability to detect the presence of certain types of cancers. This extraordinary property of dogs could lead to new tests for detecting cancer by smell. It makes you wonder if cancer has a characteristic odor?

All organisms, from the simplest bacteria to complex animals like humans, use the energy contained in food to support the incessant metabolic activity necessary for life. However, like any type of energy, this metabolism generates “waste”, products that are no longer useful to the body and must be eliminated: these are metabolites. In humans, the main elimination pathways for these wastes are the lungs (exhalation), urine and stools, and the molecular composition of these elimination pathways is necessarily greatly influenced by what we eat. Whether we think of the characteristic breath that follows a garlic-based meal, the unique smell of urine after a meal of asparagus or the strong smell of stool caused by a sudden change in the food during the first days of a trip.

All these odors are largely due to the presence of a class of molecules which are produced by metabolism and which have the characteristic of being, in a gaseous state, at room temperature. The odor receptors, present in our nose, can therefore detect the presence of these molecules in the ambient air, an odor that is often not appreciated!

Dogs have a sense of smell 1 million times more sensitive than ours

In addition to these very odorous molecules, the normal metabolism of our cells also generates a panoply of volatile compounds: a simple expiration, for example, can contain more than 3000 of these compounds! However, some studies suggest that the composition of these volatile compounds can vary depending on a person’s state of health and indicate the presence of diseases. One of the most interesting examples is cancer: during the progression of this disease, significant variations in cell metabolism cause damage (peroxidation) to the cell membrane, which leads to the production of unusual volatile compounds.

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We do not detect the presence of these molecules, because our olfactory capacities remain relatively limited and are much lower than those of certain animals, in particular canines. Indeed, dogs are capable of detecting certain odors a million times more sensitively than we are and it is this extraordinary ability that makes it possible to train certain breeds of dogs for the detection of drugs, explosives or corpses. But could these animals detect the volatile compounds characteristic of cancer?

Dogs detect several cancers by smell

Already in 1989, the British medical journal The Lancet published an article about a woman whose Dalmatian kept sniffing at a seemingly harmless stain on her leg. Intrigued, she consulted a doctor who, after analysis, told her that it was a very aggressive melanoma and that she had literally been saved by her dog!

Since that time, several studies have suggested that dogs are indeed able to detect a panoply of cancers, including breast, skin, colon and prostate, sometimes even in the initial stages of the disease. For example, a study in Germany showed that certain breeds of specially trained dogs (German Shepherd, Australian Shepherd, Labrador Retriever) were able to identify people with lung cancer: after sniffing the breath of 110 people with of cancer and 110 in health, the dogs identified 71% of the patients!

Similar results were obtained for colon cancer, where a Labrador managed to identify 91% of patients by sniffing their breath and 97% of them when exposed to their stool.

The future of cancer detection: machines sensitive to odors

Although routine clinical use of dogs for cancer detection is unlikely given the time required to train these animals, the fact remains that their feats suggest that tumors do indeed have characteristic olfactory signatures that reflect the peculiarity of their metabolism, different from other healthy cells.

The identification of molecules associated with the presence of cancer cells could therefore open the way in the future to the development of tests capable of detecting the presence of tumors simply by analyzing these molecules in the breath, urine, patient’s sweat or stool. In addition to the complementarity of the information obtained, this approach has the immense advantage of being minimally invasive for the patient.

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Williams H and Pembroke A. Sniffer dogs in the melanoma clinic? Lancet 1:734.

Ehmann R et al. Canine scent detection in the diagnosis of lung cancer: Revisiting a puzzling phenomenon. European Respiratory Journal,

Sonoda H et al. Colorectal cancer screening with odor material by canine scent detection. gut; 60: 814–819.


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