Medical Edge Newspaper Column

Proceed Cautiously on Lung-Cancer Screening

Dec. 17, 2006
DEAR MAYO CLINIC:
Which is better for detecting lung cancer -- X-ray or CT scan? Are too many X-rays or scans detrimental to your health? If you're at high risk for lung cancer, how often should you get a scan? -- Bainbridge Island, Wash.

ANSWER:
In terms of technical performance, there is no question that spiral computed tomography (CT) is better. It not only detects more cancers, but it does so when they are smaller and at relatively early stages -- hopefully before the malignancy has spread too far.

And while there is a slight risk from radiation exposure, there is no question that when a CT scan is clinically indicated, the benefit outweighs the risk. The radiation a person receives from a typical screening CT scan is about nine times that of a chest X-ray. This dose is still exceedingly small and in the range of radiation that a person gets each year from natural sources.

Yet there is quite a controversy in the medical community about the use of CT scans, as well as of X-rays, in the fight against lung cancer. Scientists question whether screening for lung abnormalities makes a difference in the individual's outcome. While some radiologists argue that CT screening should be performed on the general population as often as annually, the majority remain unconvinced. Given the fact that no scientific evidence to date has shown that screening for lung cancer by any method actually saves lives, no major health organization -- including the American Cancer Society -- recommends routine society-wide screenings.

How can these remarkable technologies fail to reduce the lung-cancer death rate, especially when they can discover tumors that are still small? For one thing, "small" may not be small enough. CT can detect abnormalities down to below one centimeter in diameter, but many tumors of this size have been developing over several years and may be aggressive enough to go on to kill the patient anyway. Alternatively, a malignant tumor may be so slow-growing that the patient would likely die from some other cause. Another complicating factor: CT scans produce many false positive results, giving rise to invasive follow-up tests and surgeries that carry risks of injury and death.

It is possible that the scientific studies performed so far have been inadequate for measuring the true value of lung-cancer screening. For this reason, the National Cancer Institute is sponsoring a major study -- the National Lung Screening Trial (NLST) -- that aims to end the controversy once and for all.

Launched in 2002 and running through 2009, the NLST follows over 50,000 current or former smokers at 30 sites (including Mayo Clinic) across the country. Unlike previous CT studies, this one uses the "gold standard" method -- a randomized controlled trial in which participants are assigned by chance to one of two screening methods. In this case, they will receive either a spiral CT or a chest X-ray once a year for three years. The resulting data will not only enable the most credible comparison yet of the two technologies, but also should determine whether either method reduces people's chances of dying from lung cancer.

Physicians take very seriously their oath of "first, do no harm" -- which explains their reluctance to routinely perform these currently questionable procedures. When the NLST's results have been analyzed, we will know with confidence whether the benefits of lung-cancer screening exceed the risks.

Although the medical community currently does not recommend routine screenings in general, neither does it recommend against selected screenings. Given that you are at high risk of lung cancer, you should learn more about the risks and benefits of testing, and discuss with your doctor whether screening would be beneficial to you.

-- Thomas E. Hartman, M.D., Radiology, Mayo Clinic, Rochester, Minn.