Web MD -- German researchers say that highly trained dogs are able to reliably sniff out lung cancer in human breath.
In its early stages, lung cancer has few symptoms, making it difficult for doctors to catch it early, when it's still treatable.
"This is the holy grail," says Suresh S. Ramalingam, MD, associate professor and director of the lung program at Emory University's Winship Cancer Institute in Atlanta.
"The whole field is focused on using something that's readily available that does not involve an expensive surgery or scan that would allow us to find early cancers," says Ramalingam, who is developing technology that aims to replicate the ability of dogs to smell trace amount of chemicals produced by cancerous tumors. He was not involved in the research.
Recently a large, government-funded study found that longtime smokers at high risk for lung cancer who received annual rapid computed tomography (CT) scans of their lungs cut their risk of dying of the disease by 20%.
But that test has caused controversy because it falsely detects cancer in about one out of four people, leading to further invasive procedures.
The new study, which is published in the European Respiratory Journal, found that four trained dogs -- two German shepherds, an Australian shepherd, and a Labrador retriever -- correctly identified cancer in 71 of 100 samples from lung cancer patients.
They also ruled out cancer in 372 out of 400 samples that were known not to have cancer, giving them a very low rate of false positives, about 7%.
"The surprising result of our study is the very high specificity of our dogs to identify lung cancer," says study researcher Thorsten Walles, MD, a lung surgeon at Schillerhoehe Hospital in Gerlingen, Germany.
"It even surpasses the combination of chest computed tomography (CT) scan and bronchoscopy, which is an invasive procedure that needs some form of anesthesia," Walles tells WebMD in an email.
Doctors have previously reported cases in which dogs have alerted their owners to undiagnosed skin, breast, and lung cancers by repeatedly pawing or nosing an affected body part. Some dogs have even been trained to smell low blood sugar levels in people who have diabetes.
But dogs have had more mixed success in carefully controlled studies, where samples from healthy people and sick people have been mixed.
A study published in BMJ in 2004 found that dogs correctly identified bladder cancer an average of about 40% of the time, a rate that was better than the 14% accuracy that could be expected by chance, but was lower than available tests.
But in June, researchers in Japan reported that dogs could detect the presence of colon cancer in human breath and stool samples with nearly 90% accuracy, a success rate only slightly lower than colonoscopy.
The length of time the dogs are trained may be an important difference between the studies, Walles says. In his study, the four dogs were trained for nine months. Other studies have used dogs trained for as little as three weeks.
The kind of sample the dogs are asked to smell -- urine, breath, blood, or stool -- may also influence the results, he says.
In his study, 220 volunteers -- 110 who were healthy, 60 who had lung cancer, and 50 with chronic obstructive pulmonary disease (COPD) -- were asked to exhale into a glass tube filled with fleece.
The tubes were mixed up so neither the dogs' handlers nor two observers who placed the samples on the floor in front of the dogs knew the status of the person they were from, to avoid inadvertently giving the dogs clues about what they should find.
The dogs were presented with five tubes at a time. Only one contained a sample from a person with cancer.
The dogs were trained to lie down and put their nose to the tube if they detected lung cancer.
The dogs appeared to be able to accurately identify the samples from cancer patients, even when they were in very early stages of the disease. And they were able to pick up the scent despite competing odors of cigarette smoke or food on a person's breath.
Researchers think dogs and other animals are able to smell disease by picking up on minute changes in compounds called volatile organic compounds (VOCs) that comprise chemical signatures in the body.
As many as 4,000 different VOCs, for example, have been identified in human breath.
A dog's sense of smell has been estimated to be 100 to 1,000 times more powerful than a human's, says Gary K. Beauchamp, PhD, director of the Monell Chemical Senses Center in Philadelphia.
"It's not just how sensitive their nose is. It's how they process this into a recognition pattern," Beauchamp says. "The reason dogs can do this is that they're recognizing a complex picture, and that's the big trick, to find out how to mimic that in some sort of device that could be useful for diagnostic purposes in human disease."
Other researchers agree.
Ramalingam says because success rates vary between dogs and between samples, the real value of knowing dogs can detect cancer will likely be in building technology that can reliably repeat what they can do.
"The dogs show that it can be done. We need to find out what the dogs are sniffing so we can do it in a more scientific manner."