Introduction

The diffuse parenchymal lung diseases (DPLD) comprise a large number of inflammatory and fibrosing pulmonary conditions, in which the pathological changes predominantly involve the alveolar parenchyma, alveolar spaces and, to a lesser degree, the peripheral airways. They can show acute, subacute or chronic presentations and can be subclassified into various groups (Figure 1). Of these, the idiopathic interstitial pneumonias are among the most problematic to diagnose for a number of reasons. The basic pathological features of the idiopathic interstitial pneumonias (IIPs) are inflammation and/or fibrosis of varying degrees and distribution. Few cases show typical specific histological features and one pattern may evolve into another over time, so there are inevitable overlaps. Furthermore, the lung parenchyma may be so scarred in advanced disease that the requisite histopathological features are only focal, or not present at all.

The main role of the pathologist in diagnosis is to classify the histological patterns of disease. He/she must then integrate this information with the clinical and imaging data to provide a final clinicopathological diagnosis, typically through multidisciplinary team (MDT) reviews. Some pathologists prefer to integrate such MDT data in their report, thereby providing a final clinicopathological diagnosis, rather than a histological pattern. A surgical lung biopsy (SLB) is generally required to provide suitable diagnostic material. However, in practical terms, where clinical and radiological features are typical of a specific interstitial pneumonia, pathological confirmation is not required for diagnosis and SLB is therefore only performed in a minority (10–20%) of cases. Patients that come to biopsy are generally those with atypical clinical findings and/or imaging at presentation, or those patients in whom there is unexpected longitudinal behavior. As histological patterns of disease may vary within and between lobes, most centers undertaking biopsies now take at least two samples from different sites, normally from different lobes. Biopsies can be from the same lobe but from areas showing different degrees of severity or different high resolution computed tomography (HRCT) appearances. This practice is based on papers showing so-called “discordance”, where different histological patterns may be present in different areas. Furthermore, it reduces the chance of sampling error and obtaining either only normal lung or only end-stage fibrotic lung. This is important as in cases with wholly end-stage fibrosis on biopsy, a diagnosis of a histological pattern of interstitial pneumonia cannot be made with confidence.