BACKGROUND. Nodules may have different lung cancer risks when new on follow-up CT versus when present on previously performed CT (i.e., existing nodules). Diameter-based Lung-RADS and volume-based NELSON (Nederlands-Leuvens Longkanker Screenings ONderzoek trial) categories have shown variable performance in nodule risk assessment. OBJECTIVE. The purpose of this study was to assess Lung-RADS and NELSON classifications of nodules detected on follow-up lung cancer screening CT examinations. METHODS. This retrospective study included 185 patients (100 women and 85 men; median age, 66 years) who underwent a lung cancer screening CT examination for which a prior CT examination was available. Stratified random sampling was performed to enrich the sample with suspicious nodules, yielding 50, 45, 47, 30, and 13 nodules with Lung-RADS categories 2, 3, 4A, 4B, and 4X, respectively. Lung-RADS categories were recorded from clinical reports. The linear measurements of the nodules were extracted from clinical reports to generate Lung-RADS categories by use of strict criteria from Lung-RADS version 1.1. Two radiologists used a semiautomated tool to obtain nodule volumes, which were used to generate NELSON categories. Lung cancer risk was assessed. ROC analysis was performed. Percentages and AUCs were weighted on the basis of Lung-RADS category frequencies in the underlying screening cohort. RESULTS. Twenty-nine cancers were diagnosed. The weighted cancer risk was 5% for new nodules, 1% for stable existing nodules, and 44% for growing existing nodules. None of the clinical Lung-RADS category 2 nodules were cancer. With use of strict Lung-RADS version 1.1 criteria, 34 nodules, including seven cancers, were downgraded to category 2. The AUC for cancer was 0.96 for clinical Lung-RADS, 0.81 for strict Lung-RADS, 0.71-0.84 for the NELSON algorithm (two readers), and 0.89 for nodule diameter measurement. Clinical Lung-RADS achieved weighted sensitivity and specificity, respectively, of 100% and 85% for the entire sample, 100% and 41% for new nodules, and 100% and 94% for existing nodules. The optimal diameter threshold was 8 mm for existing nodules versus 6 mm for new nodules. CONCLUSION. Lung-RADS, as applied by radiologists in clinical practice, achieved excellent performance on follow-up screening examinations. Strict Lung-RADS resulted in the downgrading of some cancers to category 2. Volumetric assessments had weaker performance than clinical Lung-RADS. New nodules warrant smaller size thresholds than existing nodules. CLINICAL IMPACT. The findings of the present study provide insight into radiologists' management of nodules detected on follow-up screening examinations.
Keywords: Lung-RADS; NELSON; lung cancer screening; nodule risk.