Can dogs smell cancer?


This week the Guardian reported on the ability of dogs to detect prostate cancer.

Dogs trained to detect prostate cancer with more than 90% accuracy

The Guardian – 11/04/2015

Italian scientists published a study this week that showed that two dogs they tested were able to detect prostate cancer with remarkable accuracy. One of the dogs correctly detected all cases of prostate cancer and the other detected 98.6% of cancers. On the other hand, the dogs falsely detected cancer in 1.3% and 2.4% of the negative samples too. Both dogs are German Shepherds that had previously been trained for bomb detection, and while their success as bomb sniffers was not addressed, the paper shows that as prostate cancer sniffers they are pretty amazing.

How does this compare?

In real terms, what do these numbers mean?

Consider the UK male population of roughly 30 million, with 0.15% of them being diagnosed with prostate cancer every year. That is 45,000 men.

If we test all 30 million men every year, the more accurate dog will detect almost all of the 45,000 cancers each year. However, as it also detects 1.3% false positives, it will also falsely identify the disease in 390,000 perfectly healthy men (1.3% of 30,000,000).

This number of false positives may not sound very impressive, but let’s put it in perspective: the standard lab-test for prostate cancer (PSA test) would detect around 27,000 of the 45,000 cancers, but crucially would detect a whopping 3,900,000 false positives! (The exact numbers for this depends on various variables, but I have used the estimates from here, using 3.0 ng/ml testing). So not only does it falsely detect many more prostate cancers than there really are, but importantly almost half of men that do have the disease walk away undiagnosed.

This is still a hypothetical situation as PSA is not used routinely to screen for prostate cancer in the general population, but it does emphasise just how effective these dogs are at detecting this disease. I have included a little explanation of cancer screening below for anyone who is interested.

Practicalities of using dogs in the clinic

This isn’t the first time dogs have been suggested as good cancer detectors. There have previously been reports of them detecting lung cancer, breast cancer and bladder cancer (albeit with far less impressive results than this). However, at the moment it is just not viable to introduce dogs to the clinic. A test used for cancer detection has to be reliable, and other studies haven’t proven as successful as this one. Add to that the practicalities of using live animals in the clinic (training, housing, feeding and handlers), and you can begin to see why this is not currently planned.

However, if scientists can figure out what it is the dogs are actually detecting (at the moment they have no idea), it may be possible to design a much better lab-test for it which will be far easier to get into the clinic. These “electronic noses” are already in clinical trials for lung cancer, and are showing promising results. Whether these will prove to be cheaper and more effective than dogs remains to be seen, but for the time being it is a very active and interesting area of research. And, let’s be honest, most people are likely to prefer having their urine sniffed by a dog than have a rectal examination, the current standard test for prostate problems…


Screening for cancer

Screening for a disease means testing an entire group of people for the disease, regardless of whether they show symptoms or not. The NHS in the UK provides a screening service for breast, cervical and bowel cancer. These are tests that people undergo as part of a normal health check-up, once they reach a certain age. These screens aim to flag up any potential problems, so the patients can go for further tests.

While we have tests for many other cancers, these are the only three that it is deemed cost effective to screen for. Take the PSA test for prostate cancer, mentioned above. The cost to the NHS to further test the high number of false positives would be immense. On top of that there is the worry and stress experienced by people who test positive wrongly. There has been debate in the medical field as to whether breast cancer screen is worth doing, for the same reason.

Obviously it would be very desirable to screen the population for every cancer. The earlier a tumour is caught, the better. However, the tests we have for the large majority of cancers are either not reliable enough, or are too expensive, for screening purposes.

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