Diverse histopathological patterns in Fleischner-defined interstitial lung abnormalities: Radiologic-Pathologic...

Interstitial lung abnormalities (ILA) are CT-detected interstitial abnormalities that may represent early-stage interstitial lung disease (ILD). However, histopathologic correlations remain limited, with previous studies reporting conflicting results. In 2025, the American Thoracic Society (ATS) established criteria defining the boundary between ILA and ILD. This study aimed to investigate the histopathological spectrum of Fleischner-defined ILA, correlate findings with CT features and outcomes, and reclassify cases according to 2025 ATS criteria.

This retrospective study analyzed 30 consecutive patients who underwent surgical lung biopsy between January 2010 and December 2021. All cases had ILA per Fleischner Society criteria and were reclassified using 2025 ATS criteria. Three pulmonary pathologists evaluated dominant and co-existing histopathological patterns. Two chest radiologists independently assessed CT findings. Overall survival was compared between usual interstitial pneumonia (UIP)-related and non-UIP-related groups using Kaplan-Meier analysis.

Twenty patients (66.7 %) were men; mean age was 63.6 ± 6.5 years; 27 (90 %) had fibrotic ILA. The most common dominant pattern was UIP (43.3 %), followed by nonspecific interstitial pneumonia (NSIP) and bronchiolocentric interstitial pneumonia (BIP) (20 % each). Overall, 96.7 % of cases met 2025 ATS criteria for ILD (subclinical ILD). On CT, all UIP and NSIP cases showed reticular opacity and traction bronchiectasis, while BIP typically exhibited branching linear opacities. During median follow-up of 92.1 months, overall survival did not differ significantly between UIP-related and non-UIP-related groups (p = 0.595).

This study of Fleischner-defined ILA, predominantly reclassified as subclinical ILD by 2025 ATS criteria, demonstrates diverse histopathological patterns beyond UIP, including BIP, highlighting pathological heterogeneity.