Fig. 5

CT Alternative Diagnosis with corresponding histopathology UIP with acute exacerbation. a Thin slice CT shows asymmetric, non-zonal fibrosis and extensive ground glass opacity classified as Alternative Diagnosis by the 2018 ATS and Fleischner guidelines. Although the radiologic differential diagnosis included acute exacerbation of UIP, interpretation was challenging due to difficulty in classifying the underlying pattern of fibrosis as UIP b–c Corresponding histopathology from the subsequent surgical lung biopsy shows all features of UIP required by both guidelines, but with some superimposed organizing pneumonia (arrows). The amount of organizing pneumonia was not considered to be ‘prominent’, as required by the guidelines to move the classification to Alternative Diagnosis, so the case was classified as UIP but was favored to be in an accelerated stage. This case demonstrates a limitation in the application of the guidelines, where subjective features quantifiers such as ‘prominent’ result in an apparent disagreement in CT and histologic guideline categorization when the primary diagnosis is in agreement. Hematoxylin and eosin (H&E) stain. Scalebar, panel B: 0.5 mm. Scalebar, panel C: 0.1 mm