Small pulmonary nodules: reproducibility of three-dimensional volumetric measurement and estimation of time to follow-up CT.

Abstract

To determine reproducibility of volume measurements of small pulmonary nodules on computed tomographic (CT) scans and to estimate critical time to follow-up CT. One hundred fifteen pulmonary nodules for which two thin-section small-field-of-view CT scans were obtained and which were stable during 2-year observation were evaluated. A standard group of 94 nodules (with no or minimal artifact) and an expanded group of 105 nodules (including those with moderate artifacts) were examined. Percentage volume change (PVC) and monthly volumetric growth index (MVGI) were computed for each nodule pair. By using estimates of the variation in PVC in stable nodules as a function of initial diameter, critical time to follow-up CT was estimated; this time is the earliest point at which growth in a nodule of a given size can be reliably identified with repeat CT. The SD of PVC decreased with increasing nodule size from 18.5% in 2-5-mm nodules to 10.6% in 5-8-mm nodules and to 7.47% in 8-10-mm nodules. Inclusion of cases with moderate motion artifacts increased the SD of PVC to 27.4% in 2-5-mm nodules, to 17.1% in 5-8-mm nodules, and to 19.3% in 8-10-mm nodules. Critical time to follow-up CT for nodules detected at baseline screening was 12, 5, and 3 months and 1 month for those with initial sizes of 2, 5, 8, and 10 mm, respectively. For nodules detected at annual repeat screening, it was 4 and 3 months and 1 month for nodules that were 3, 4, and 5 mm or larger in size, respectively. Mean MVGI in 94 standard cases was 0.06%, and standard error was 0.21%. Factors that affect reproducibility of nodule volume measurements and critical time to follow-up CT include nodule size at detection, type of scan (baseline or annual repeat) on which the nodule is detected, and presence of patient-induced artifacts.