Confluent fibrosis and fibroblast foci in fibrotic non-specific interstitial pneumonia

AimsThe separation of fibrotic non-specific interstitial pneumonia (FNSIP) and usual interstitial pneumonia (UIP) is important for patient treatment and prognosis, but is sometimes a difficult diagnostic problem. Most authors believe that fibroblast foci are rare in NSIP, and that the finding of multiple fibroblast foci suggests a diagnosis of UIP. Similarly, architectural distortion is viewed as favouring a diagnosis of UIP. This study aims to assess these criteria for their diagnostic utility. Methods and resultsWe report eight patients with a high-resolution computed tomography diagnosis of FNSIP, and a picture of fibrotic NSIP on biopsy, but in whom, in all cases, fibrosis focally widened the walls to the point of confluence, producing architectural distortion in the form of variably sized, sometimes quite large, blocks of fibrosis, but not honeycombing. Fibroblast foci varied from four to nine per section, and point counting morphometry showed that the areas containing large blocks of confluent fibrosis were geographically strongly associated with fibroblast foci, whereas fibroblast foci were rare away from the confluent fibrosis. Follow-up imaging studies (mean interval 19.5 months; range 2-42 months) in six patients revealed stable or improved disease in four and worsening disease in two. No case had or developed radiological honeycombing, and there were no other imaging findings to suggest a diagnosis of UIP. ConclusionsOtherwise typical FNSIP cases can show architectural distortion caused by confluence or marked expansion of fibrotic alveolar walls. These areas tend to be associated with fibroblast foci. These findings do not imply a poor prognosis, and should not be confused with UIP.