For the first time, national guidelines are recommending that high-risk adults be screened for Canada's deadliest cancer, and the tool of choice is a technology known as low-dose computed tomography, or CT scanning.

The Canadian Task Force on Preventive Health Care says adults from the ages of 55 to 74 with a history of significant tobacco use should be tested for signs of lung cancer annually over three consecutive years using the sensitive radiological imaging device.

The guideline applies to current smokers and those who have quit within the past 15 years, who have at least a so-called 30 pack-year history of smoking – defined as smoking one pack a day for 30 years or two packs a day for 15 years.

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"Screening for lung cancer aims to detect disease at an earlier stage, when it may respond better to treatment and be less likely to cause serious illness or death," said Dr. Gabriela Lewin, chairwoman of the task force's working group on lung cancer screening.

The guideline is based on recent studies, primarily a U.S. clinical trial that assessed patients after seven years of followup and found a 15-per-cent reduction in lung cancer deaths among participants screened via CT scans for three years in a row, compared with those tested with chest X-rays.

Lung cancer is Canada's most common malignancy and the No. 1 cancer killer. In 2015, about 26,600 Canadians were diagnosed with lung cancer, and almost 21,000 died from the disease.

"Most lung cancers are not symptomatic until they have advanced to late stages of the disease and are incurable," Dr. Natasha Leighl, president of Lung Cancer Canada, said in a statement. "That's why it's encouraging to see these guidelines, which recommend a screening test that could lower mortality – representing a chance to save lives."

The recommendations do not apply to people with a family history of lung cancer or those with symptoms suggestive of lung cancer, which include persistent cough, coughing up blood or rust-coloured phlegm, shortness of breath, and chest pain that worsens with deep breathing, coughing or laughing.

For people with other risk factors, such as exposure to radon gas, second-hand smoke or previous chest radiation, it's not known whether there is any benefit to screening with low-dose CT, say the authors, whose guidelines were published Monday in the Canadian Medical Association Journal. Lewin said studies on those populations have not yet been done.

"So for all other adults, regardless of age and smoking history or any other risk factors, we recommend not to screen for lung cancer with low-dose CT," she said from Kemptville, Ont., where she practises family medicine.

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Among the reasons for limiting screening to those 55 to 74 with a lengthy history of smoking is that CTs can produce a proportion of false positives, meaning the scan picks up what appears to be a nodule in the lungs, but it isn't actually malignant.

That can lead to such further testing as additional CT scans and/or a bronchoscopy – a procedure in which a flexible tube with a lens is inserted to look inside the lungs' airways – and a lung biopsy, said Lewin, adding that such invasive testing can cause bleeding, a collapsed lung, hospitalization or even death, "all for trying to find the cancer earlier."

In some cases, CT scans can lead to "overdiagnosis" by detecting cancers that would never cause symptoms in the person's lifetime, meaning that the patient would have died with the cancer, not because of the cancer, she said.

Heather Bryant, vice-president of cancer control at the Canadian Partnership Against Cancer, said the guidelines represent the first time new advice can be offered for lung cancer screening on the weight of scientific evidence.

"It's especially helpful in lung cancer," said Bryant, noting that the disease kills more Canadians than breast, colorectal and prostate cancers combined.

"And yet, we haven't made a lot of progress in survival for this disease," she said. The average five-year survival rate for those diagnosed with lung cancer is 17 per cent, based on 2006-2008 estimates by the Canadian Cancer Society.

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No provinces or territories have formal CT screening programs for lung cancer, although Alberta has a pilot project in the works and "we know that people will be working on developing guidelines," Bryant said.

"However, we also know that with other screening (programs), if you don't plan before it gets out into practice, it can be very difficult to deliver it with the care and quality that is needed," she said.

Questions include how often screening should be performed, how to handle positive scans, and whether screening can benefit those with other lung cancer risk factors – questions that can only be answered by carefully assessing the results as screening proceeds, she said.

"This is why we're saying that regardless of what we do, we need to make sure this is done in places that not only have high-quality and well-trained individuals, but that we have agreement on the kind of information that we'll need to collect to evaluate this as we go forward."

Screening must be paired with smoking-cessation programs to support people who are still using tobacco, added Bryant, who also heads the Pan-Canadian Lung Cancer Screening Network, which has developed a framework to help provinces implement CT-scan programs aimed at early detection of lung cancer.