More than 18% of all lung cancers detected by low-dose computed tomography (CT) appeared to represent an overdiagnosis, according to a recently published study.

Low-dose CT has been shown in recent clinical trials to be an effective screening tool in some patients, but some of the tumors it finds may be indolent (slow growing) or clinically insignificant. Overdiagnosis is the detection of a cancer with a screening test that wouldn’t otherwise have become clinically apparent. It is a potential harm of screening because of the additional cost, anxiety, and complications associated with unnecessary treatment. The study was published in JAMA Internal Medicine (2013; doi:10.1001/jamainternmed.2013.12738).

Edward F. Patz Jr, MD, of Duke University Medical Center in Durham, North Carolina, and colleagues, examined data from the National Lung Screening Trial, which compared low-dose CT screening versus chest radiography among 53,452 people at high risk for lung cancer, to estimate overdiagnosis.

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Among 1,089 lung cancers reported in the low-dose CT group during follow-up, the authors estimated that 18.5% represented an overdiagnosis. They also estimated that 22.5% of non-small cell lung cancer detected by low-dose CT represented an overdiagnosis, and that 78.9% of bronchioalveolar (air sacs) lung cancers detected by low-dose CT represented an overdiagnosis.

The use of low-dose CT to screen for lung cancer showed a 20% relative reduction in mortality, and 320 individuals needed to be screened to prevent one lung cancer death. The authors cautioned that, although these findings were met with enthusiasm, the risks of screening also need to be considered. They stated that, “it is not clear that all early-stage lesions detected in asymptomatic individuals will progress to cause symptoms and affect long-term outcome.”

“In the future, once there are better biomarkers and imaging techniques to predict which individuals with a diagnosis of lung cancer will have more or less aggressive disease, treatment options can be optimized, and a mass screening program can become more valuable,” the authors concluded. They explained that overdiagnosis should be considered when the risks of low-dose CT screening for lung cancer are described.