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Evidence of Benefit

continued...

A less familiar harm is overdiagnosis,[27] the diagnosis of a condition that would not have become clinically significant had it not been detected by screening. In the case of screening with LDCT, overdiagnosis could lead to unnecessary diagnosis of lung cancer requiring some combination of surgery (e.g., lobectomy, chemotherapy, and radiation therapy). Although overdiagnosis is almost impossible to document in a living individual, autopsy studies suggest that many individuals die with lung cancer rather than from it. In one study, about one-sixth of all lung cancers found at autopsy had not been clinically recognized before death.[40] Even this may be an underestimate because autopsy probably fails to detect many small lung cancers that are detectable by CT.[41] Studies in Japan provide additional evidence that screening with LDCT could lead to a substantial amount of overdiagnosis.[42] In a study in which smokers and nonsmokers were annually screened for lung cancer between 1996 and 1998 using LDCT, the overall rate of screen-detected lung cancers was very similar in the two groups: 0.46% for smokers (mainly men) and 0.41% for nonsmokers (mainly women).[43] The nonsmoking group may have included individuals who were at an elevated risk for lung cancers for other reasons, but no information is provided on this point. A second study involving both smokers and nonsmokers reported a similar finding of a 1.1% lung cancer detection rate in both groups.[44] Confirmative studies are needed to establish the level of overdiagnosis that might be associated with CT screening for lung cancer. In that same population, the volume-doubling times of 61 lung cancers were estimated using an exponential model and successive CT images. Lesions were classified into three types: (1) type G (ground glass opacity), (2) type GS (focal glass opacity with a solid central component), and (3) type S (solid nodule). The mean-doubling times were 813 days, 457 days, and 149 days for types G, GS, and S, respectively. In this study, annual CT screening identified a large number of slowly growing adenocarcinomas that were not visible on chest x-ray.[45] Before spiral CT is accepted into medical practice, it is critical to determine whether this modality does more good than harm in a randomized controlled trial with lung cancer mortality as the endpoint.[46,47]

To assess the feasibility of conducting a randomized controlled trial in asymptomatic individuals at high risk of lung cancer, the NCI conducted the Lung Screening Study (LSS). Between September 5, 2000 and November 15, 2000, six PLCO contract screening centers recruited 3,318 heavy or long-term smokers (inclusion required a 30 pack-year smoking history) who were not participants in the PLCO trial and randomly assigned them to receive a baseline and 1-year LDCT (1,660) or chest x-ray (1,658). The two study arms were essentially identical on age, sex, and history of smoking. Compliance with screening declined from 96% at baseline to 86% at 1 year in the LDCT arm and from 93% at baseline to 80% at 1 year in the chest x-ray arm. In a survey of all study patients who had greater than a 98% response rate, 2.6% of chest x-ray patients reported having a CT exam outside the trial between annual screens, and 13% of LDCT patients had outside chest x-rays. Positivity rates in the LDCT arm were 20.5% at baseline exam and 25.8% at the 1-year screen; the chest x-ray arm, positivity rates were 9.8% and 8.7%, respectively. Positivity rates were higher among current smokers and older patients. Lung cancer was diagnosed in 1.9% of participants in the LDCT arm at baseline and 0.57% at year 1; in the chest x-ray arm, lung cancer was diagnosed in 0.45% and 0.68% of participants, respectively. The cumulative probability that a participant would receive at least one false-positive test during the study was 33% for LDCT and 15% for chest x-ray.[48] Forty cancers in the LDCT arm (48% were stage I) and 20 in the chest x-ray arm (40% were stage I) were diagnosed during the study period. A total of 16 stage III to stage IV cancers were observed in the LDCT arm versus nine in the chest x-ray arm. Almost all patients with positive screening results received at least one follow-up diagnostic procedure (98% in the LDCT arm and 96% in the chest x-ray arm). This information proved the feasibility of the National Lung Screening Trial (NCI-NLST).[49,50]

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Last Updated: October 07, 2011
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