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There are a lot of stories out there about dogs who seemed to smell lung cancer on their owner’s breath, and a recent study found that some dogs can detect lung cancer in blood samples with astonishing accuracy. So why aren’t there domestic dogs trained to detect cancer in every hospital on the planet?


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Sources:
https://www.americanveterinarian.com/news/cancersniffing-dogs-where-are-we-today
https://www.economist.com/science-and-technology/2019/03/28/scientists-discover-the-chemicals-behind-the-unique-parkinsons-smell
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https://www.medicalnewstoday.com/articles/323620.php
https://www.ncbi.nlm.nih.gov/pubmed/22012770
http://flipper.diff.org/app/items/6954
https://www.mdpi.com/1424-8220/11/1/1105/htm
https://www.cancer.org/cancer/breast-cancer/screening-tests-and-early-detection/mammograms/limitations-of-mammograms.html
https://www.scientificamerican.com/article/if-dogs-can-smell-cancer-why-dont-they-screen-people/?redirect=1
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6042482/
https://www.popsci.com/problem-with-cancer-sniffing-dogs
https://www.bbc.com/news/magazine-26472225
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https://erj.ersjournals.com/content/34/1/261
http://goodcanineacademy.org/index.php/cancer-sniffing-dogs/
https://www.thelungcancerproject.org/screening/?c=ale-16aa37935e0&gclid=CjwKCAjw8-LnBRAyEiwA6eUMGng06lXr7I_7TO2FyoBrBF7YygjIlxV6R5E2_svS-ONiltY2k-9hxRoCiHEQAvD_BwE&gclsrc=aw.ds
https://www.news-medical.net/health/What-is-a-Biomarker.aspx
(Intro)

Right now, as we speak, there exists a relatively inexpensive method for detecting certain early cancers with an accuracy rate of up to 97%.  You might even have one in your room with you right now, and it probably goes wild when you reach for a tennis ball or the doorbell rings.  Yes, we're talking about the domestic dog.  So why aren't there domestic dogs trained to detect cancer in every hospital on the planet?  It's complicated.

Anecdotally, there are lots of stories out there about dogs who seemed to smell lung cancer on their owner's breath or became unusually interested in a particular mole that was later found to be cancerous, but there's also more rigorous evidence, too.  In fact, a 2019 study found that some dogs can detect lung cancer in blood samples with almost 97% accuracy, and that's not too surprising when you consider that dogs can learn a huge amount about other dogs' health, age, and reproductive status by smelling their butts, but how they're able to separate the smell of healthy cells from cancerous cells is pretty remarkable.

Cancer cells produce odiferous compounds called volatine organic compounds or VOCs and dogs sometimes actually detect a particular cancer's signature blend.  The particular group of VOCs produced by a given type of cancer can act as a biomarker for that cancer.  A biomarker is any biological characteristic that can be measured and then used to identify some sort of process in the body.  If that sounds vague, it's because biomarkers can be a huge range of things, but in this case, they make it possible to tell healthy tissue from cancerous cells.

VOCs are produced during normal biological processes.  They might be present in skin, urine, blood, sweat, stool, or even a person's breath, where they can serve as biomarkers for both healthy and diseased biological processes.  That's because the VOC profile you see in these places is different in people with, say, lung cancer than it is for healthy people, and it's different for people with lung cancer than it is in people with ovarian cancer or bladder cancer or breast cancer. 

Detecting cancer-specific VOCs is complicated for us humans because there are so many to sort through.  There can be as many as 3,000 different VOCs in a person's healthy exhaled breath.  In 2003, researchers were able to identify two classes of VOCs: alkanes and monomethylated allkanes as possible tumor markers in lung cancer patients.  The specific biomarkers of many other cancers haven't been identified yet.  That's mostly because it's challenging to pick out the few VOCs that are specific to a given cancer from all of the other given compounds that might be present in a sample, but if that 2019 study is any indication, some of our fine furry friends can be trained to sniff them out.

So you'd think that every hospital and doctor's office and diagnostic lab that doesn't already have a cancer-sniffing dog would be trying to get one but there are some technical barriers despite these pooches' impressive accuracy.  Dogs thrive in jobs that require a close relationship with a handler.  Dogs that sniff out drugs or bombs or who look for survivors in disasters work in a very engaging environment.  These jobs are exciting for dogs.  The animals are working in an envrionment with a lot of external stimuli and that moment where a survivor is found under a pile of rubble can be, like, emotional for everyone involved, and a search and rescue dog's handlers can confirm that the dog has found its target, which means the dog will always get the right reinforcement at the right time.

Compare that to a bunch of dogs in a lab sniffing out sample after sample, most of which probably won't be positive.  That can be frustrating for a dog, but the biggest problem in that real-life scenario is that the handlers can't give the dogs any feedback if they don't know which samples are cancerous.  If they reward the dog for every alert, they could unknowingly be rewarding false positives, which will ultimately impact the dog's accuracy.  No rewarding means no joy and those cancer-sniffing canines could easily start regretting their career choice, and even though the dogs can be pretty accurate in theory, things could be different in practice. 

Like people, dogs are fallible.  I don't wanna--don't send me hate mail, and unlike a hypothetical cancer sniffing machine, a dog might work better in the morning and not so great in the afternoon, which could result in different rates of accuracy.  You don't have that kind of problem with a machine which operates the same way every time you use it.  

Finally, it's a huge investment in both time and money, to train a cancer-sniffing dog.  A single dog needs to spend 6-8 months sniffing a minimum of 300 biological samples before they can be a certified cancer-sniffer, but that doesn't mean we can't learn a lot from cancer-sniffing canines.  Dogs can still help us identify the odor signatures of different cancers.  We can use that information to build electronic noses, robot cancer-sniffing dogs.  Like, not literally, but it was fun to say.

These devices are already being tested but more work needs to be done to determine which VOCs the noses should be looking for as well as to improve their overall accuracy.  So far, some studies are using electronic noses to look at patterns of VOCs rather than picking out specific ones, because the specific VOCs aren't always known, and electronic noses still aren't as accurate as some dogs are.  

Other medical conditions, for example, can sometimes interfere with the machine's ability to spot cancer.  One study using VOCs to detect bladder cancer found that the presence of an infection could reduce the accuracy of the electronic nose, which means that there's more progress to be made.  Still, while it's fun to imagine legions of lab coat wearing canines quietly saving lives behind the scenes, it's probably not gonna happen in quite that way, but the things we've learned from dogs and their amazing olfactory abilities are already helping us build technologies that will save lives through early diagnosis.

So break out that tennis ball and thank your best friend for all of their help.  They might not be sniffing out cancer, but they're real cute and sweet and they like you.

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