Pre-existing interstitial lung disease, emphysema, pattern of pneumonitis, and extent of pneumonitis were determined by two experienced pulmonologists and an experienced radiologist based on CT findings. Pre-existing interstitial lung disease did not include radiation pneumonitis. In each case, CT patterns of pneumonitis were classified according to a previous report15 (link) as (1) DAD pattern; (2) nonspecific interstitial pneumonia pattern; (3) hypersensitivity pneumonitis pattern; (4) OP pattern; and (5) simple PEo pattern16 (link). Furthermore, we divided CT patterns of pneumonitis into DAD and non-DAD according to the irAE guideline11 (link). The extent of pneumonitis was divided into < 25%, 25–50%, and > 50% of the lung parenchyma12 (link), and the typical images of patients in our cohort are shown in Fig. 2.

Drug-related pneumonitis showing new diffuse ground-glass opacities, consolidation, and traction bronchiectasis, indicative of a diffuse alveolar damage pattern (a). Drug-related pneumonitis showing new ground-glass opacities, irregular reticular opacities, and irregular reticular opacities with predominant lower lung involvement, indicative of the nonspecific interstitial pneumonia pattern (b). Drug-related pneumonitis showing new wide areas of faint ground-glass opacities with some patchy nodular lesions (arrowheads), indicative of the hypersensitivity pneumonitis pattern (c). Drug-related pneumonitis showing new ground-glass opacities and consolidations with multifocal distribution, indicative of the organizing pneumonia pattern (d). Drug-related pneumonitis showing new focal opacity areas (arrowheads). Lesions disappear only with withdrawal of drug therapy (not shown here), with features compatible with the simple pulmonary eosinophilia pattern (e).

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