The results of a recently published study of low-dose CT screening in lung cancer carried out by a collaborative group of American researchers suggest that one in five tumors detected by CT scans are probably so slow-growing that they would never cause problems (Patz et al . Overdiagnosis in low-dose computed tomography screening for lung cancer. JAMA Intern Med. 2014;174(2):269. doi: 10.100). The group analyzed data that had been accumulated in the US National Lung Screening Trial (NLST). 
This is the randomized trial which compared screening using low-dose computed tomography (LDCT) vs chest radiography (CXR) among 53 452 persons at high risk for lung cancer observed for 6.4 years. In the NLST trial half of the people received three annual low-dose CT scans —and half got three annual conventional chest X-rays. During six years of follow-up, 1,089 lung cancers were diagnosed in CT scan patients, versus 969 in those who got chest X-rays. The group calculated two measures of overdiagnosis. The first was the probability that a lung cancer detected by screening with LDCT was an overdiagnosis (PS), defined as the excess lung cancers detected by LDCT divided by all lung cancers detected by screening in the LDCT arm of the trial The second measure was the number of cases that were considered overdiagnosis relative to the number of persons needed to screen to prevent 1 death from lung cancer. Such overdiagnosis cases represent an important potential harm of screening because they incur additional cost, anxiety, and morbidity associated with cancer treatment.
The lesions detected were not false-positives — they were indeed cancerous tumors, but ones that caused no symptoms and were unlikely ever to become deadly. The problem is that there is no easy means to determine which symptomless tumors found on CT scans might become dangerous, so automatically all cancers are aggressively treated.
“The findings underscore the need to identify biological markers that would help doctors determine which tumors are harmless and which ones require treatment” said Dr. Edward Patz, lead author and a radiologist at Duke University Medical Center.
The influential US Preventive Services Task Force recently issued a draft proposal recommending annual CT scans for high-risk current and former heavy smokers. A final recommendation is pending, but the Task Force had already assumed that screening might lead to overdiagnosis. The American College of Radiology has also published recommendations for CT screening of high – risk lung cancer subjects showing that such screening significantly reduces lung cancer deaths in high-risk patients. The ACR said that the benefit “significantly outweighs the comparatively modest rate of overdiagnosis” found in the new analysis.