A positive correlation between nodule size and the probability of malignancy has been clearly demonstrated (5-10). The commonly used standard size value is the average of the maximum and minimum cross-sectional diameters of the most representative areas of the nodule. A meta-analysis based on eight large census trials showed that the probability of malignancy depended on nodule size: 0% to 1% when nodules were 5 mm or smaller, 6% to 28% when nodules were between 5 and 10 mm in size, and 64% to 82% when nodules were 20 mm or larger. Even among smokers, the percentage of all nodules smaller than 4 mm that ultimately prove fatal is very low (<1%), while the percentage of nodules in the 8 mm range is about 10%-20%. The Fleischner Society 2005 guidelines state that at least 99% of all nodules 4 mm or smaller are benign. Such small shadows are common on thin CT scans and therefore CT follow-up is not recommended for every case; a single follow-up scan within 12 months should be considered for cases with suspicious nodule morphology or high risk.