Maybe my math is goofy, but with NNT=320, does that mean, if these results are generalizable, that it took 1000 CT scans (with the consequent 125 false positive work ups) to save one cancer death?
In November 2010, the NIH announced results from the National Lung Screening Trial (NLST): Low-dose computed tomography (CT) screening of smokers lowered lung cancer–related mortality. These results have now been published.
About 53,000 current and former smokers (age range, 55–74; smoking history, at least 30 pack-years) were randomized to annual lung cancer screening for 3 years by either low-dose CT or conventional chest x-ray (CXR). Noncalcified nodules measuring ≥4 mm were considered to be positive, as were abnormalities such as effusions and adenopathy. Findings were communicated to participants and their physicians; a specific protocol for further evaluation was not mandated.
During median follow-up of 6.5 years, lung cancer–specific mortality was significantly lower in the CT group than in the CXR group — 1.33% vs. 1.66%. Deaths attributed to invasive diagnostic procedures and cancer treatments were considered lung cancer–related deaths. The number needed to screen to prevent 1 lung cancer death was about 320.
False-positive screenings were common: 39% of participants in the CT group and 16% in the CXR group had at least one positive screen, and 95% of those results were false-positives. Diagnostic follow-up of positive CT screenings included roughly 8800 standard CTs, 2500 chest x-rays, 1500 positron-emission tomography scans, 320 percutaneous biopsies, 670 bronchoscopies, and 710 surgeries. Among those with positive CT screening results, major complications of invasive diagnostic procedures occurred in about 12% of patients in whom cancer was eventually diagnosed and in <1% of patients in whom cancer was not diagnosed.
This study demonstrates that CT screening can lower lung cancer–related mortality. However, many questions remain. For example, will radiologists generally be able to duplicate the performance of NLST study radiologists? In the community, will low-dose CT (as opposed to standard-dose CT) be readily available and will evaluation and follow-up of screen-positive patients maximize benefit and minimize harm? Given the high false-positive rate, how should we weigh the costs and morbidity of screening and its sequelae? Because of these and other questions, the authors and an editorialist conclude that policymakers should wait for cost-effectiveness analyses and other information before endorsing screening. I agree with that conclusion. (For additional information about the NLST, see Table.)