Characteristic computed tomography features of mild-to-moderate Legionella pneumonia: evaluation of a known pattern and identification of a novel indicator

This study aimed to re-evaluate the accuracy of known computed tomography (CT) findings and identify potentially novel radiological features to facilitate the accurate diagnosis of Legionella pneumonia. A total of 18 cases of genetically identical Legionella pneumophila serogroup 1 pneumonia from a single outbreak were retrospectively analyzed. The chest CT scans were assessed for known and novel imaging features. Associations between CT findings and clinical scores, indicated by A-DROP and Legionella scores, were evaluated. Validation was performed using historical Legionella pneumonia cases (n = 4) and cohorts of Streptococcus pneumoniae (n = 22) and Mycoplasma pneumoniae pneumonia (n = 30). A CT pattern described as sharply demarcated peribronchovascular consolidation with ground-glass opacity was identified in nine patients (50.0%).Plate-to-fan-shaped consolidation extending from the peribronchovascular area to the pleura, a newly recognized feature, was observed in six patients (33.3%). At least one of these findings was observed in each of the 11 cases (61.1%), particularly in those with A-DROP scores of ≤ 2. These features were significantly more frequent in patients with Legionella pneumonia than in those with pneumonia caused by S. pneumoniae or M. pneumoniae. The interobserver agreement level was substantial to almost perfect. Characteristic CT patterns were frequently observed in mild to moderate Legionella pneumonia and may assist in early radiologic diagnoses, especially in cases where clinical suspicion is low.