Lung CT scan for cancer: Should you be screened?

David Midthun, M.D.

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If you're a smoker or former smoker, you may worry about your risk of lung cancer. At this time, there's no proven way to effectively screen for lung cancer in current and former smokers. But researchers are studying ways to identify lung cancers early — when they're more likely to be cured. One focus of research is computerized tomography (CT) scanning of the lungs.

Whether lung CT scans are helpful when it comes to diagnosing early lung cancer has yet to be proved. Here, David Midthun, M.D., a consultant in the Division of Pulmonary and Critical Care Medicine at Mayo Clinic, Rochester, Minn., explains the pros and cons of using lung CT scans for lung cancer screening.

With some cancers, doctors encourage people to get regular screening. Why is it different with lung cancer?

Screening for breast, cervical and colon cancers has been shown to reduce your risk of dying of these diseases when compared with those who don't get screened. Doctors recommend screening for these cancers because doing so has proved beneficial for a group of people in the study setting. This same standard should be met for lung cancer before screening is broadly recommended.

Although there have been hopeful developments with CT scan screening for lung cancer, at this point there isn't convincing evidence that screening results in more good than harm. In fact, a study of current and former smokers showed that lung CT scan screening detected three times as many cancers as expected. Screening also resulted in 10 times as many surgeries as expected and did not reduce the number of people with advanced cancer or the expected deaths from lung cancer. So the bottom line is lung CT scan screening did not appear to help.

Isn't all cancer screening good? Isn't it better to know that you have lung cancer than to not know?

According to the studies, lung CT scans help doctors diagnose more lung cancers, but this isn't necessarily a good thing. Diagnosing more cancer is only beneficial if the number of advanced stage cancers is reduced and, as a result, fewer people die of lung cancer. So far, studies have shown that lung CT scans do not decrease the number of advanced stage cancers found or decrease risk of dying of lung cancer.

Simply diagnosing more cancer may not be beneficial. Some of these cancers are so slow growing (indolent) that they would never have caused symptoms or death. Others are so aggressive that treatment would not change the outcome.

Won't lung CT scans help doctors diagnose lung cancer earlier?

Although studies have shown that screening increases detection of early-stage cancer, there is still not sufficient evidence that those diagnoses end up being more beneficial than harmful or that they reduce the likelihood of death.

Being diagnosed with lung cancer at an earlier stage does make it more likely that surgery — the best treatment for most types of lung cancer — can remove the cancer. We certainly advocate for someone diagnosed with lung cancer to undergo surgical treatment if appropriate. But at this point, lung CT scan screening hasn't been shown to save lives.

What are the risks of lung CT scans?

CT scans spot abnormalities in about 20 to 60 percent of smokers and former smokers, but most of these abnormalities are scars from smoking, inflammation or other noncancerous conditions. The CT scan's sensitivity can result in doctors suspecting cancer where there really is no cancer, which results in invasive follow-up tests, unnecessary surgery, and anxiety for the patient and their loved ones.

CT scans are so good at seeing nodules or spots on the lung that about half of those screened will have one or more nodules. A majority of these nodules are tiny and don't need immediate testing, but they do need follow-up CT to detect any changes.

Larger nodules require other tests to help find out whether cancer is present. This leads to repeat scans, biopsies and sometimes surgery, all of which carry some risk. In other words, screening leads to necessary invasive tests and surgery for lung cancer, but these tests end up being unnecessary if it's eventually determined that the spot first found on a CT scan isn't lung cancer but some other benign process.

Also, CT scans expose you to a small dose of radiation. Compared to some other parts of the body, such as the breast, lungs have greater potential for developing radiation-induced cancer.  Several studies have estimated an increased risk of developing cancer from CT scan radiation. The risk of developing cancers from CT scans is small, but it's a reminder of the importance of weighing the risk vs. the benefit of any medical test. Until studies prove that CT lung cancer screening saves lives, this risk is an additional caution to consider.

What do you tell patients who ask about lung CT scans?

It's important to be wary of claims that you can improve your chances of survival with lung cancer through CT scan screening. Screening will increase survival statistics just by telling more people that they have cancer, even though the screening may not lead to those people living longer than they would have without the screening. You want to know that screening will reduce the likelihood of dying of lung cancer, and this hasn't been proved. Studies that can determine this are under way, and results may be available in the next few years. Conclusions from one large study, the National Lung Screening Trial (NLST), are expected in 2010.

Currently, no major medical organizations, including the U.S. Preventative Services Task Force recommend screening for lung cancer. On the basis of the evidence and these recommendations, Mayo Clinic doctors aren't screening for lung cancer on a routine basis with any tests. In the absence of demonstrated benefit from CT scan screening, we are focusing on risk reduction in trying to get smokers to stop smoking.