Answer: Usual interstitial pneumonitis (UIP) pattern

Histology: none provided

Discussion: Microscopic characteristic features in this open lung biopsy are “subpleural accentuation of disease, interstitial inflammation and fibrosis with a pattern of “temporal heterogeneity”. That is, there are areas of lung which are essentially normal in appearance interspersed with areas of fibrosis and other areas of interstitial inflammation. In addition, there is honeycomb change with remodeling of air spaces and lining by cuboidal, bronchial type cells, and there are scattered “fibroblast-foci”. These foci are composed of areas of relatively “young” collagen and are recognized on H&E stains by their pale appearance as opposed to the deep Eosin staining of “mature” collagen. Fibroblast foci are randomly scattered throughout the lung parenchyma. When they occur in a small airway, they may be confused with Bronchiolitis Obliterans Organizing Pneumonia. The random distribution of fibroblast foci and their appearance away from small airways will resolve the issue. The temporal heterogeneity of the UIP pattern is the primary reason that transbronchial lung biopsies are usually inadequate for accurate diagnosis. The diagnosis of the UIP Pattern is based on the topography and variation of the lesion in the affected lung. NSIP, or Nonspecific Interstitial Pneumonitis is a distinctly different lesion. In cases of NSIP, fibroblast foci are unusual, established interstitial fibrosis is not a characteristic feature, and neither is established interstitial fibrosis with honeycombing. Perhaps more importantly, the lesion of NSIP is temporally homogenous and should not vary from area to area in the affected lobe. The UIP Pattern is a characteristic lesion in the lung of patients with the clinical and radiographic diagnosis of Idiopathic Pulmonary Fibrosis. The pattern, however, is not specific, and may be seen in patients with collagen vascular disease and, more importantly, can to some degree occasionally be seen in patients with asbestosis. In this case, there was no history of exposure to asbestos and iron stains were negative for asbestos bodies. In Bronchiolitis Obliterans Organizing Pneumonia (BOOP), the areas resembling “fibroblast foci” are located only at the airway/airspace interface. There may be interstitial inflammation and even some interstitial chronic inflammation in the septa near airways involved by BOOP but the pattern should fade away the further one gets from the airway. Again, clinical and radiographic correlation will resolve the issue in most instances.

Reference(s):
– Katzenstein A L: Katzenstein and Askin’s Surgical Pathology of Non-Neoplastic Lung Disease, Third Edition, W.B. Saunders Co., 1997.
– Gal AA, and Staton G.W., Jr. American Journal of Clinical Pathology 2005; Suppl.,S67-S81.