Report from ASCO: False positives crop up frequently in CT lung cancer screening

June 1, 2009

Low-dose CT lung cancer screening carries a high burden of false-positive results after only two rounds of testing, according to a presentation at the 2009 American Society of Clinical Oncology.

Low-dose CT lung cancer screening carries a high burden of false-positive results after only two rounds of testing, according to a presentation at the 2009 American Society of Clinical Oncology.

CT screening for lung cancer has been marketed to the public without any clear evidence that it is beneficial, said lead investigator Dr. Jennifer Crosswell.

"We've seen some increasing promotion of low-dose CT scans as a lung cancer screening tool to the public," she said during an ASCO press conference. "This is mostly being done by individual hospitals and some advocacy organization groups ... suggesting that the value of these scans is to alleviate anxiety among smokers and former smokers. We don't have good evidence that if you get screened, you'll actually see a reduction in lung cancer deaths."

Crosswell and colleagues from the National Institutes of Health and the National Cancer Institute in Bethesda, MD, sought to estimate the cumulative risk of an individual receiving at least one false-positive screening results. They also looked at whether these false positives promoted invasive procedures.

They analyzed data from a feasibility pilot study performed before the ongoing National Lung Screening Trial was launched. The NLST enrolled 3190 participants who had baseline CT scans or chest x-rays followed by one repeat annual scan. The subjects, both current and former smokers, were followed for one year after final screening. A scan was deemed a false positive if it led to a complete negative workup or more than 12 months of follow-up with no cancer diagnosis.

Using Kaplan-Meier analysis, Cromwell's group determined that the cumulative probability of at least one false-positive test for an individual was 21% (95% confidence interval, 19% to 23%) after one screening study and 33% (95% CI, 30% to 35%) after the second screening test. On multivariable analysis, higher odds of false-positive results were associated with increased participant age (64 years and older) and current versus former smoker status.

The investigators also analyzed the diagnostic follow-up for false-positive findings and noted that 61% of those participations with false-positive results on low-dose CT scan had to undergo additional imaging. Another 6.6% underwent invasive procedures.

Crosswell said that physicians and patients should discuss the cumulative risk for false-positive results.

"Given that we see such a high burden of false positives with low-dose CT lung cancer screening, we feel that careful investigation of the physical, psychological, and economic ramifications is warranted," she said.

Dr. Julie Gralow, director of breast medical oncology at the Seattle Cancer Care Alliance, University of Washington, noted while moderating a news conference for the study that the true value of any cancer screening test is an improvement in survival.

"We need to wait for the NLST results so that we can see whether there is a survival benefit," she said. "If there is, then we'll accept a certain number of false positives."