P1.11-30 Very Rapid Growth of Small Pulmonary Nodules Predicts Benignity

Growth of pulmonary nodules on repeat CT is used to identify malignant lesions, although very rapid growth is thought to imply an inflammatory process. Few data exist examining the optimum threshold at which rate of growth predicts a benign aetiology. Using an institutional CT database of small (<15mm) solid pulmonary nodules (n=784), we identified patients with antecedent (≥30 days prior) thin section (≤2mm) CT imaging and a final diagnosis of primary lung malignancy or a definite benign diagnosis based on pathology or longitudinal CT follow up data (n=137). Enlarging nodules (volume growth >25%) were identified (n=63) using semi-automated volumetry, and the volume doubling time (VDT) calculated. In cases where no nodule existed on the antecedent CT, a volume of 5mm3 was assigned, permitting the calculation of a ‘virtual’ VDT. Comparison of volume doubling time between benign and malignant nodules was made using Wilcoxon signed rank test. A receiver operator curve was constructed, and the optimum threshold of nodule growth rate predictive of benignity was calculated using the methods of Miller. The final study population consisted of 63 nodules in 57 patients [32/62 (50.8%) malignant, median age 67 years (range 34–85 years), male = 30/57 (52.6%)]. There was no difference in patient age nor in smoking status between groups, although patients with malignant diagnoses significantly more likely to be female (p <0.001). The median time between baseline (T1) and antecedent (T0) scans was 260 days (interquartile range 343 days). At baseline (T1), benign lesions (median diameter 10mm, median volume 380 mm3, range 10-4300mm3) were significantly smaller than malignant nodules (median diameter 13mm, median volume 890mm3, range 60-4250 mm3); p = 0.001. 24/31 benign lesions and 3/32 malignant lesions were not visible on the T0 scan, and were assigned a volume of 5mm3. The median benign lesion VDT was 70 days (interquartile range 270 days), malignant median VDT was 188 days (interquartile range 170 days); p = 0.2. The majority of lesions with very rapid growth (VDT <90 days) were benign diagnoses (n= 17/24 [70.8%]). When examining these rapidly growing nodules, the optimal cut-point of the receiver-operator was a VDT of 50 days, AUC = 0.735. This provided 100% specificity for benign disease. Our results confirm that very rapid nodule growth predicts benignity; a VDT of <50 days was 100% specific for benignity. Further work is required to validate these findings in other cohorts.