Lung CT Scans Produce False Alarms

1 in 3 CT Screens for Lung Cancer Detection Produce False-Positive Results

Medically Reviewed by Louise Chang, MD on June 01, 2009
From the WebMD Archives

June 1, 2009 (Orlando) -- One in three people who undergo serial CT scans to spot lung cancer are given false-positive results that can lead to unnecessary -- and potentially harmful -- follow-up tests, government researchers report.

The false alarms also cause needless anxiety that can have a negative impact on mental and physical well-being, says study head Jennifer M. Croswell, MD, acting director of the NIH Office of Medical Applications of Research.

The findings come at a time when many hospitals are promoting CT scans for the early detection of lung cancer, particularly in smokers and ex-smokers, she says.

“One ad that really bothered me said, ‘Quit smoking? Now quit worrying. Have a scan,’” Croswell tells WebMD. “In fact, there’s a reasonable probability that [the scan] will have the opposite effect.”

It’s the invasive follow-up exams that really bother Peter G. Shields, MD, deputy director of the Lombardi Comprehensive Cancer Center in Washington, D.C.

“A one-in-three chance [of having a false positive] is huge, even if anxiety is the only negative effect. But the results can lead to invasive tests that cause pain and suffering. That’s unacceptable,” he tells WebMD. Shields did not work on the study.

CT Scan Produces Twice as Many False Alarms as X-rays

The new study involved more than 3,000 current or former smokers aged 55 to 74. About half got CT scans and half got standard chest X-rays. A year later, everyone got a second exam, using the same screening test they got the first time. Then they were followed for another year.

The study was presented at the annual meeting of the American Society of Clinical Oncology.

The second CT scan produced false-positive results for cancer in 33% of patients. That’s more than twice the 15% false-alarm rate associated with X-rays, Croswell says.

A false positive was defined as findings that indicated a suspicion of cancer that were later found to be noncancerous by biopsy, repeat scan, or at least 12 months of follow-up with no cancer diagnosis.

Of the patients with false positives on CT, nearly 7% had a more invasive diagnostic test such as a biopsy or bronchoscopy, in which a scope is used to look down the airway to see if there is a mass.

Nearly 2% had lung resection or other major surgery. “As with any surgery, there is a risk of complications, such as blood loss and infection. And there’s also a small but real risk of death,” Croswell says.

“Even a biopsy can end up causing a collapsed lung,” she says.

CT Scans: False Positives Lead to Repeat CT Scans

The majority of patients whose CT results later turned out to be false positives -- 61% -- were scheduled for repeat CT scans.

That might not sound so bad, but “many people don’t want to wait two or three months for another test. The idea of waiting drives them crazy. If there’s cancer, they want it out now,” Shields says.

The researchers also looked at whether certain factors, such as age or being a current vs. former smoker, placed people at greater risk for false positives on CT. The only factor that appeared to raise the odds of getting a false alarm was being over age 64.

Shields says that one of the problems is that doctors don’t yet know whether CT screening for lung cancer actually save lives.

Two large-scale studies -- the U.S. National Lung Screening trial and the European NELSON trial -- aimed at answering that question are under way. Results could be available as early as next year.

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American Society of Clinical Oncology Annual Meeting 2009, Orlando, May 29-June 2, 2009.

Jennifer M. Croswell, MD, acting director, NIH Office of Medical Applications of Research.

Peter G. Shields, MD, deputy director, Lombardi Comprehensive Cancer Center, Georgetown University, Washington, D.C.

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