Malignant solitary pulmonary nodules: assessment of mass growth rate and doubling time at follow-up CT

Abstract

Background: The differentiation of benign and malignant solitary pulmonary nodules (SPNs), especially subsolid nodules, is still challenging because of the small size, slow growth, and atypical imaging characteristics of these nodules. We aimed to determine the significance of mass growth rate (MGR) and mass doubling time (MDT) at follow-up CT of malignant SPNs.

Methods: This retrospective study included 167 patients (169 SPNs, diameter 8-30 mm). Among the 169 SPNs, 114 malignant SPNs were classified into three types: pure ground-glass nodules (pGGNs), part-solid nodules (pSNs), and solid nodules (SNs). These patients were followed up for at least 3 months. Three-dimensional manual segmentation was performed for all these nodules, and the intra- and inter-observer variabilities of diameter, volume, and mass measurement were assessed. From initial and follow-up CT scans, growth rates of the diameter, volume, and mass of the SPNs were compared. MDT and volume doubling time (VDT) were calculated and were compared among groups.

Results: Mass measurements had the best inter-observer consistency and intra-observer repeatability; the coefficients of variation of the mass measurements were the smallest. The mean growth rates of the diameter, volume, and mass of pGGNs, pSNs, and SNs significantly differed at different time points (P<0.001). Mean MDTs and VDTs of pGGNs, pSNs, and SNs were 655 vs. 848 days, 462 vs. 598 days, and 230 vs. 267 days, respectively (P<0.05).

Conclusions: Mass measurements are an objective and accurate indicator in SPN assessment. During a 2-year follow-up, the mean growth rates of the diameter, volume, and mass of pGGNs, pSNs, and SNs differed at different time points, the greatest difference was observed in mean MGR. Mean MDT of malignant SPNs is less than the mean VDT.

Keywords: Solitary pulmonary nodule (SPN); lung cancer; mass doubling time (MDT); mass growth rate (MGR); volume doubling time (VDT).

Figure 1

Enrollment and exclusion of subjects. SPN, solitary pulmonary nodule; GGN, ground-glass nodule

Figure 2

A-66-year-old man with a history of SPN in the right upper lobe. Diameter, volume, and mass of nodule were measured and analyzed at the baseline and during follow-up at 3, 6, 12, 18, and 24 months. Section thickness =2 mm.

Figure 3

Inter-observer and intra-observer Bland-Altman plots of measurement variability in the 169 SPNs. SPN, solitary pulmonary nodule; SD, standard deviation.

Figure 4

Mean growth rates of diameter, volume, and mass of different types of nodules with different follow-up intervals (%). SPN, solitary pulmonary nodule; GGN, ground-glass nodule.