Lung cancer screening often yields false positives

Imaging tests used to screen symptom-free people for lung cancer often find suspicious growths that turn out to not be cancer, according to a U.S. government study published Monday.

Researchers say the findings point to a significant downside of using the tests -- chest X-rays or spiral CT scans -- to try to detect lung cancer early.

In recent years, CT scans, in particular, have been promoted by some hospitals and advocacy groups for lung cancer screening, even though studies have not yet shown whether such screening saves lives.

"The most important thing right now is to try to figure out if this lowers death rates," Dr. Jennifer M. Croswell, the lead researcher on the new study, told Reuters Health.

The current findings, she said, add "pieces to the puzzle" by estimating how likely a person is to have a false-positive result if he or she has repeat screening tests over time.

For the study, Croswell and her colleagues at the National Institutes of Health (NIH) followed nearly 3,200 smokers and former smokers who were randomly assigned to undergo either a chest X-ray or CT scan to detect potentially cancerous abnormalities. If the first result was normal, participants had another test one year later.

Overall, the study found, the chances of having a false-positive result -- a suspicious lesion that turned out not to be cancer -- were 21 percent after one CT scan, and 33 percent after two.

With chest X-rays, there was a 9 percent chance of a false-positive after one test, and a 15 percent chance after two, the researchers report in the Annals of Internal Medicine.

The problem with false-positives is that they can lead to needless follow-up procedures, as well as stress and anxiety for patients and their families.

That follow-up may include invasive tests, which carry some risks. More often, false-positive become apparent when patients have a follow-up CT scan or chest X-ray anywhere from a few months to a year later.

Those extra imaging tests are a concern, according to Croswell, because of the added radiation exposure. The CT scans are known as "low-dose" CT, but a single scan still carries a much higher radiation dose than a single chest X-ray, Croswell noted.