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.

It is not yet known, she said, what the long-term risks of radiation exposure from repeated lung cancer screening might be.

In this study, 7 percent of participants with a false-positive CT result had an invasive follow-up procedure; most commonly, that meant a bronchoscopy, where a thin, camera-bearing tube is passed through the nose or mouth and threaded into the lungs’ airways.

But some patients had more-invasive procedures, such as a lung or lymph node biopsy. Eight ended up having major surgery for what turned out to be benign conditions.

Of study participants with a false-positive chest X-ray, 4 percent went on to have an invasive procedure.

The findings come from the pilot phase of an ongoing NIH clinical trial looking at whether CT or chest X-ray screening cuts death rates among people at increased risk of lung cancer. The trial, which began in 2002, ultimately enrolled more than 53,000 current and former smokers, and the results could become available in the next couple years, Croswell said.

Despite the current lack of proof that screening prevents deaths, certain hospitals and advocacy groups have been promoting CT scans, which cost anywhere from $300 to $1,000 per test, for lung cancer screening.

Both chest X-rays and spiral CT scans can detect small tumors in the lungs; the CT scans, which generate three-dimensional images of the lungs, are especially sensitive - detecting growths that are well below 1 centimeter in size. However, such abnormalities are frequently not cancer.

“Most professional medical societies do not recommend this (lung cancer screening) right now,” Croswell said, “and the reason is because we don’t know if it works.”

For smokers and former smokers who are nonetheless interested in lung cancer screening, Croswell said that they should be aware that there are “important downsides.” She suggested that they talk with their doctors about all the potential benefits and risks before making a decision.

However, Croswell pointed out, the most important thing a smoker can do about his or her lung cancer risk is to quit.

SOURCE: Annals of Internal Medicine, April 20, 2010.