Screening people for lung cancer ought to work. Lung cancer is common, relative to other cancers, so you're not looking for a needle in a haystack. If it's caught at a very early stage, it often can be treated effectively with surgery and even cured. Current and former smokers are easy to identify, so focusing on a high-risk group is pretty straightforward. And the technological means to spot suspicious growths and lesions in the lungs has existed for decades with chest x-rays and CT scans
Screening tests are supposed to find diseases before they cause symptoms, but that is just the means to an end. The goal is for fewer people to die from the disease. Several large, well-designed trials of chest x-rays have found that while they do a good job of identifying early lung cancers, that hasn't translated into fewer deaths because by the time an x-ray detects lung cancer it has almost always already spread to other parts of the body.
CT scanning can spot lung cancer at an earlier stage than chest x-rays. But can they spot lung cancer early enough, before it has spread, and when it is still curable? The jury has been out because of the size and design of studies that have been done so far. It takes a very large, well-designed study to get a reliable answer.
An important study
One such large, well-designed study has now been done. In November 2010, the National Cancer Institute (NCI) announced preliminary results from the largest randomized trial of lung cancer screening ever conducted. The study, called the National Lung Screening Trial (NLST), included over 53,000 current and former heavy smokers, ages 55 to 74, who had no signs or symptoms of lung cancer when they agreed to be in the study. The results showed that more than twice as many lung cancers (649 vs. 279) were discovered in the people screened with CT scans as with chest x-rays.
But early detection wasn't the point of this trial. It was set up to find out whether the screening method made a difference in the number of deaths from lung cancer. So researchers followed the two groups for several years, keeping track of who died — from lung cancer and from other causes.
During the follow-up period of about five years, 354 of the people in the study screened with CT scans (roughly 1.4%) died of lung cancer. Over the same period, 442 of those screened with chest x-rays (roughly 1.7%) died of the disease. There were also 7% fewer deaths from all causes in the CT scan group compared with the chest x-ray group. So even though the death rate from lung cancer was slightly lower in the group that had CT scans, it isn't clear that the CT scans were the reason for the lower death rates.
The sheer size of the NLST makes these findings important. The fact that the NLST had lung cancer deaths as its main outcome is also crucial. Cancer screening trials that use other outcomes, such as how long people survive after a diagnosis, can be misleading. Showing improvements in survival after diagnosis, without showing whether the number of deaths has been affected, may mean a screening test is just moving up the time at which a diagnosis is made, instead of leading to early treatment that truly lengthens life.
Time to get screened?
So, should heavy smokers, present or past, get a CT scan of their lungs? It's too soon to make a recommendation. The full results of the trial haven't even been reported yet. The NCI has said publication in a peer-reviewed journal is scheduled for some time in 2011.
But it will take months, if not longer, for groups like the American Cancer Society and the U.S. Preventive Services Task Force to mull over the results and issue guidelines. Medicare and other insurers may wait to make coverage decisions until those groups weigh in.
If screening with CT scans does become the norm, people will have to be warned about the very real possibility of false positives. Almost one in every four people who were screened with CT scans in the NLST had a false positive, compared with about one in every 14 in the x-ray group. A false positive result leads to fear and anxiety that, in retrospect, was unnecessary, and also sometimes leads to invasive tests, such as biopsies that cause risks and, in retrospect, also were unnecessary.
Another concern is added radiation exposure. If suspicious findings lead to follow-up scans, the amount of radiation exposure could start to add up.
Every current or former heavy smoker knows that he or she is at risk for getting lung cancer. If a test could catch it at an early, curable stage, such a test would be welcome. This study suggests that CT scanning might catch cancers at a curable stage in some patients, but also shows that it catches many lung cancers after they no longer are curable. So, as with any cancer screening test, you have it in hopes that you will be the person in whom the cancer has been caught when it still is curable, but knowing that there is no guarantee that it will.
The psychological comforts should not blind us to some of the problems with cancer screening. Results from the NLST suggest that about 300 heavy smokers will need to be screened with CT scans, at a cost of perhaps several hundred dollars each, to prevent just one death from lung cancer over a five-year period. That ratio actually compares pretty well to screening mammography, especially when used for detecting breast cancer in younger women. But it's still a reminder that screening is inherently an effort that affects many to benefit a few. From a public health perspective, it may be that the dollars spent on CT scan screening of heavy smokers could be better spent on helping people to quit — or preventing them from smoking in the first place.
Overdiagnosis is possibly another problem. Screening tests (PSA screening for prostate cancer is a prime example) can lead to detection of slow-growing cancers that would never have caused illness. Lung cancers don't seem to linger in an indolent state, so screening with CT scans isn't expected to cause a big overdiagnosis problem, but it isn't out of the question, either. Longer-term follow-up of the people in the NLST may shed some light on the issue.
March 2011 update
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