ACR Lung-RADS guidelines for pure ground-glass nodules: is a threshold of 20mm adequate?

Purpose: The ACR Lung-RADS for management of pulmonary nodules categorizes pure ground-glass nodules (GGN) into a 1 year (category 2) or 6 months (category 3) follow-up CT scan, using a cutoff value of 20mm as threshold. An analysis of GGNs categorized according to the Lung-RADS is presented. Methods and Materials: All pure GGNs from two sites of the NELSON screening trial were selected. Morphology was determined in the first CT scan they were annotated. Each nodule was considered only once. Diameter measurements were taken from the NELSON database. Nodules were categorized according to the ACR Lung-RADS, using the proposed threshold of 20mm. Changes in categorization were calculated when lowering the threshold to 15 and 10mm. Results: 86 GGNs were included, with 16 malignant GGNs (19%) including 9 invasive adenocarcinomas. For a 20mm threshold, only 2% of GGNs were assigned to Lung-RADS 3. For a 15mm threshold this increased to 16%; for a 10mm threshold to 53%. Of the 16 cancers, only 1 was assigned to Lung-RADS 3 using a 20mm threshold. This increased to 4 and 12 for thresholds of 15 and 10mm, respectively. Conclusion: Using a threshold of 20mm to select pure GGNs that warrant shorter follow-up will miss almost all cancers. A less conservative threshold of 10mm - as has been suggested by the Fleischner Society - will select three-quarters of the cancers at the expense of an overall increased number of followup CTs.