![]() In an attempt to overcome these various difficulties, groups sponsored by the Medical Research Council in the UK and the National Cancer Institute in the USA are currently piloting prospective, randomized, controlled trials of 40,000 and 88,000 patients respectively using low-dose CT. Individuals in the control-arm trials may worry that they are missing out on optimal treatment and manoeuvre their way into the screened population. Cross-contamination between the screened and control arms of the study is also a problem in these large trials, especially as the public at large become more aware of health issues. This lack of so-called “stage shift” again questions the ability of low-dose CT screening to decrease overall mortality. ![]() Although low-dose CT can detect early stage disease 6–10 times more frequently than chest radiography 11, 15, there has not as yet been a similar fall in the prevalence of advanced disease 2. Change in mortality rather than survival is necessary to validate such screening methods 2. Although survival from the time of diagnosis of the disease is commonly reported it is not an appropriate measure of a diagnostic screening test and may be misleading as it is subject to lead-time bias, length-time bias and overdiagnosis bias. The importance of rigorous study design cannot be overemphasized when assessing the validity of these large and expensive trials. Although this seems low, it should be remembered that breast-cancer screening has a detection rate of only 0.6–0.7% 14. In this study, the cancer detection rate was 2.7% but it was <0.5% for the two other published studies (table 1 ⇓). However, by assessment of patterns of calcification at both low-dose and high-resolution CT (HRCT) and repeat scanning after an interval, the Early Lung Cancer Action Project (ELCAP) group had only one incidence of biopsy performed for a benign, noncalcified nodule 11. The logistics of differentiating benign from malignant nodules therefore becomes a very real issue and there have been concerns about the number of biopsies that may need to be performed. In the Mayo Clinic trial 7 for example, over one-half of all patients had at least one nodule. However, only a small percentage of these nodules turn out to be lung cancer. These trials show that CT detects many more lung nodules than chest radiography. Also included in the table is preliminary data from two ongoing trials in the USA and Germany. Their findings are summarized in (table 1 ⇓). ![]() In the last 5 yrs three nonrandomized trials incorporating low-dose computed tomography (CT) have reported prevalence screening data 11– 13. Critically, however, none showed a statistically significant reduction in overall mortality. All showed increased detection of early-stage lung cancer, more resectable cancers and improved 5‐yr survival rates in the screened versus control groups. Why is screening not performed? Three large American screening programmes in the 1970s sponsored by the National Institute of Health 6– 9 and another in Czechoslovakia in the 1980s 10 screened high-risk populations using chest radiography and sputum analysis. The main risk factor, smoking, is easily identifiable and noninvasive screening tests such as chest radiography and sputum cytology are widely available. Despite this, lung cancer has an overall prognosis so dismal that incidence exceeds prevalence 5. The disease is very common and in its earliest stages ≤70% of cases can be cured by surgery 4. Lung cancer, in theory, should lend itself to screening. ![]()
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