Dogs can be trained to detect early and late stages of lung and breast cancer accurately according to a study published by California scientists in the little-known scientific journal Integrative Cancer Therapies.
The study took place over the last five years at the Pine Street Foundation, a non-profit organization which conducts evidence-based research on integrative medicine (combining complementary and alternative medicine and mainstream medicine). Michael McCulloch and colleagues used three Labrador Retrievers and two Portuguese Water Dogs, both common pets, that received basic behavioral dog training. The researchers trained the dogs to lie down next to a sample from a cancer patient and to ignore other samples.
The samples used were breath samples from 55 patients with lung cancer and 31 with breast cancer — the two types of cancer with the highest mortality rates in the United States.
After the training phase, the dogs' accuracy diagnosis was tested in a double-blind experiment. Among lung cancer patients, the sensitivity and specificity were 99% accurate and for breast cancer sensitivity was 88% and specificity 98%. Because these figures seem almost too good to be true, cancer experts are the same time baffled and skeptical. The authors of the study themselves also say replication of the study is needed.
Importantly, this was independent of the cancer stage, meaning the dogs were able to pick up the scent of cancer in its early stages. This is important because in many cases, the success of any treatment depends on early diagnosis. However, the researchers don't believe this will lead to the use of dogs in the clinic soon, rather they want to find out which chemicals are actually sensed by the canines, because they could be used in laboratory assays. "It's not like someone would start chemotherapy based on a dog test," Dr. Gansler of the American Cancer Society said, "They'd still get a biopsy.".
The researchers were inspired by anecdotal reports about dogs detecting cancer. In 1989, a British women consulted with her family physician because her Dalmatian kept licking a mole on her leg. At biopsy it showed to be malignant melanoma. When diagnosed too late this form of cancer has a poor survival rate, but in this case early surgery was made possible, and the women survived. Prior studies showed that breath samples from patients with lung cancer or breast cancer contain distinct biochemical markers. This provides a basis for the hypothesis that some cancer types produce volatile chemicals that dogs could smell. A study published in the British Medical Journal already proved that dogs could use their exquisite sense of smell to detect bladder cancer in urine samples, but they were only correct in 41% of cases, and another study provided preliminary evidence that dogs could detect melanomas.
This doesn't mean you can show your breasts to your dog and it will tell you if you have cancer, other physicians caution, and scientists do not advise people to train their dogs to sniff for cancer. Unresolved issues from the study include the fact that subjects were required to breathe deeper than normal, so it's not sure whether dogs can smell cancer in normal breath. Also, whether this is a permanent skill that would be retained by dogs was not tested. 
Finally, there are concerns that could arise over liability issues: who would be responsible when the dog makes a mistake?
Current detection methods for both lung and breast cancer are not flawless. For lung cancer, chest X-ray and sputum cytology (detecting cancer cells in coughed up fluid) fail to detect many early cases, and CT scan produces many false-positive results unless combined with expensive PET scans. Although it might be comparing apples and oranges, a $2.5 million CT scanner has an accuracy of 85 to 90%. Mammography also produces false-positive results, and it may be difficult in women with dense breast tissue. As such, another type of "pet"-scan, using dogs as a biological assay, might prove feasible for screening if supported by further research. Current tests are also expensive so the use of dogs for preliminary cancer testing could prove to be an affordable alternative for countries in the developing world.
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Michael McCulloch, Tadeusz Jezierski, Michael Broffman, Alan Hubbard, Kirk Turner, and Teresa Janecki. Diagnostic Accuracy of Canine Scent Detection in Early- and Late-Stage Lung and Breast Cancers. Integrative Cancer Therapies, 2006; 5(1): 30-9. PubMed
Study says dogs can smell lung and breast cancer