Abstract
Diseases of the air space of the lung share many common findings and presentation that it makes more sense to be grouped together in a differential diagnosis. The common imaging finding is usually as a lung infiltrate or consolidation of some type of distribution or another. Trying to find the putative cause for the imaging pattern is a good first step in narrowing the differential diagnosis and answering the question: what accounts for this pattern on imaging studies? Most of these diseases have an acute or subacute onset and exhibit restrictive pattern on pulmonary function tests. When treated promptly, they resolve completely with little or no residual injury to the lung architecture. On a small biopsy it is important to have a clinical history and imaging results before embarking on a descriptive rather than a definitive diagnosis. The smaller the sample, the more information is needed to complete the picture. Pulmonologists should be advised to target the part of the lung that should provide the most information related to the process under investigation. An endobronchial wall biopsy is not suitable in the setting of investigating a lung infiltrate as part of the alveolated lung has to be included. The category of airspace-occupying diseases of the lung includes: 1. Acute lung infections such as bacterial and viral pneumonias. 2. Acute lung injury pattern which includes bronchiolitis obliterans/organizing pneumonia and diffuse alveolar damage. 3. Intra-alveolar hemorrhage. 4. Pulmonary alveolar proteinosis. 5. Pulmonary edema. 6. Pneumocystis pneumonia. 7. Lymphangitic carcinomatosis. In all of these instances, there are ground-glass opacities obscuring the air space. Most clinicians would describe the imaging as “infiltrate” or “consolidation.” The task for the pathologist is to find the pertinent finding that could account for the imaging.