Schaner and colleagues²⁸ reported that CT defined more nodules than conventional tomography in 48% of cases of clinically diagnosed metastatic disease to the lung for a variety of tumors including osteogenic sarcoma, Ewing’s sarcoma, rhabdomyosarcoma, fibrosarcoma, and melanoma. The additional nodules were usually pleural or subpleural and 3 to 6 mm in diameter. Siegelman and associates²⁹ examined 634 solitary pulmonary nodules detected by CT. They found that 113 of 634 of these were <1 cm in size and 32 (28.3%) of these 113 were malignant. Of these 32, 18 were primary pulmonary malignancies and 14 were metastases. Of tumors between 1.1 and 2.0 cm in size, 149 (46.9%) of 318 were malignant; and of those >2.0 cm, 174 (85.7%) of 203 were malignant.

Kaneko and colleagues⁸ introduced low-dose spiral CT into screening for lung cancer, which increased the percentage of early-stage lung cancer detected at screening. Before the introduction of CT, 43 lung cancers were detected in 26,338 screening examinations and 18 (41.9%) of 43 lung cancers were stage IA. After the introduction of CT, 36 lung cancers were detected in 9,993 examinations, 28 (77.8%) of which were stage IA. The mean size of the lung cancers detected by CT alone was 11.5 mm. Henschke and colleagues⁷ recruited 1,000 high-risk individuals for baseline and annual repeat CT screening. At last follow-up, a total of 1,184 annual repeat screenings had been performed, among which 7 cases of malignancy were found. The median size of these was 8 mm.

Nawa and associates²¹ performed screening using low-dose spiral CT as a part of the annual health examinations of a total of 7,956 individuals. A total of 2,865 noncalcified solitary pulmonary nodules were detected, and 40 patients (41 lesions) were found to have lung cancer. Thirty-five of these 41 tumors were stage I. Swensen and colleagues³⁴ reported a prospective cohort study comprising 1,520 individuals. Two years after baseline screening, 2,832 uncalcified pulmonary nodules were identified in 1,049 participants (69%) and 40 cases of lung cancer were diagnosed. The mean size of the 37 NSCLCs detected at CT was 15.0 mm. Bastarrika and associates¹ studied 150 asymptomatic smokers using nonenhanced low-dose spiral CT; they found 54 noncalcified pulmonary nodules of 141 symptom-free subjects. The diameter was ≤5 mm in 24 participants (70.6%), 6 to 10 mm in 7 (20.6%), and >10 mm in 3 (8.8%). One patient with a noncalcified pulmonary nodule of ≥10 mm underwent a complementary positron emission tomography (PET) examination, which was positive. Biopsy of this nodule demonstrated lung cancer (squamous cell carcinoma).

Maximum-intensity-projection (MIP) processing reduces the number of overlooked small nodules, particularly in the central lung. Observer nodule detection remains imperfect even when lesions are clearly depicted on images. Gruden and associates⁶ retrospectively identified 25 patients with metastatic disease, each having from 2 to 9 nodules that were 3 to 9 mm in diameter. The addition of MIP slabs significantly enhanced reviewer
detection of central nodules (P <0.001) and junior reviewer detection of peripheral nodules (P <0.001).

Lee¹⁷ conducted a retrospective review over a 15-year period to identify patients with surgically resected NSCLC measuring ≤1 cm. The majority of the tumors were discovered incidentally by CT or chest radiography. Togashi³⁵ retrospectively investigated 62 operated tumors measuring ≤1 cm. All tumors were detected by medical checkup.

Swensen and associates³⁴ have observed a significant CT artifact in the evaluation of lung nodules that occurred with the use of algorithms using high spatial frequency reconstruction. They concluded that use of this modality for the analysis of lung nodules with thin-section CT may lead to an erroneous diagnosis of calcification. Kimme-Smith¹⁰ and colleagues quantified differences in the detection of simulated lung nodules on CT scans on the basis of variations in nodule size, local contrast, body habitus (global contrast), and exposure. On such scans, small nodules (5 mm in diameter) can be reliably detected when they are located in areas of high or moderate surrounding local contrast, such as the lung or mediastinal regions. Detection of nodules decreases in regions of lower optical density corresponding to the subdiaphragmatic regions of the chest. The decrease in nodule detectability is greatest under conditions of large body habitus and underexposure.

Wormanns and colleagues³⁸ reported that CT-aided diagnosis improves the detection of pulmonary nodules after spiral CT and offers a valuable second opinion in lung cancer screening despite its still limited sensitivity. The algorithm is designed to detect lesions at least 5 mm in diameter.

Although postinflammatory change is the most common lesion presenting as pulmonary nodules, calcification is the most definitive finding for benign inflammatory lesions. Yankelevitz and Henschke³⁹ wanted to use the relationship between the true and apparent nodular density on 10-mm CT sections to determine the true density of a nodule. An understanding of the partial volume effect of small pulmonary nodules on 10-mm CT images made it possible to identify accurately those that were diffusely calcified without having to resort to obtaining additional 1-mm CT sections. To determine whether the likelihood of lung nodule calcification can be predicted from nodular size as measured on a chest radiograph, Ketai and colleagues⁹ retrospectively reviewed chest radiographs of patients with lung nodules ≤1 cm in size that were detected on CT scanning. They concluded that nodules detected on chest radiographs that were <7 mm in size were likely to be calcified or to represent a false-positive finding. Intrapulmonary lymph nodes often present as pulmonary nodules. Kinoshita¹¹ and Fujimoto⁴ and their associates reported lymph nodes detected by CT that could not be diagnosed by bronchoscopic examination or CT-guided needle biopsy. They subsequently required video-assisted thoracic surgery (VATS) to be diagnosed pathologically. To assess the use of early repeat CT of solitary pulmonary nodules to determine nodular growth, Yankelevitz and colleagues⁴⁰ performed repeat CT scans as part of their routine clinical protocol. Preliminary experience with early repeat CT suggested that a single repeat CT scan obtained 30 days after the first scan can depict growth in most malignant tumors as small as 5 mm. Yankelevitz and colleagues⁴¹ also recommended CT volumetric measurement, which is highly accurate for determining volume and useful in assessing the growth of small nodules and calculating their doubling times.