Characteristics of small lung cancers invisible on conventional chest radiography and detected by population based screening using spiral CT

Conventional chest radiography (CXR) is a poor diagnostic tool for detecting lung cancers at a surgically curable stage. To determine the visibility of peripheral small lung cancers on CXR, we retrospectively examined the usefulness of CXR using a consecutive series of 44 cases detected on CT screening and later confirmed by histopathology. All cases had been detected by low dose CT during a population based screening trial for lung cancer. The control group consisted of 48 chest radiographs of normal subjects. Tumour diameters ranged from 6 mm to 45 mm, with 95% (42/44) less than or equal to 20 mm, and 5% (2/44) >20 mm. CXR failed to detect 77% (34/44) of all cancers, including 79% (33/42) less than or equal to 20 mm and 50% (1/2) >20 mm. Of the 42 lung cancers less than or equal to 20 mm, 74% (31/42) were located in the well penetrated lung zones and 71% (22/31) of these were missed on CXR. 26% (11/42) were concealed by hilar vessels, mediastinum, heart or diaphragm, and all (11/11) of these were missed on CXR. 93% (39/42) of the lung cancers less than or equal to 20 mm were adenocarcinomas and 79% (31/39) of these were missed on CXR. 7% (3/42) were epidermoid carcinomas or small cell carcinomas and 66% (2/3) of these were missed on CXR, The overall accuracy of interpretation on CXR for lung cancers was 61%, sensitivity was 23% and specificity 96%. Although there was an association between presence of lung cancer and positive reading of CXR (chi(2) test of association, p<0.05), the percentage of positive readings was only 23%. Thus, CXR was poor at visualizing CT detectable lung cancers of less than or equal to 20 mm diameter, which are usually of very low density, and cannot be relied upon for detection of surgically curable small lung cancer.