Reactive Type II Pneumocyte Hyperplasia Versus Adenocarcinoma

Reactive type II pneumocyte hyperplasia is a nonspecific reactive proliferation of type II pneumocytes in response to a variety of lung injuries and to repair and restore alveolar epithelial integrity. Type II pneumocyte hyperplasia, when exuberant, can exhibit overlapping cytomorphology with adenocarcinoma. The underlying lung injuries sometimes can present as a mass lesion clinically. In the setting of limited biopsy specimens, it is important to differentiate reactive type II pneumocyte hyperplasia from adenocarcinomas with lepidic or acinar growth patterns to avoid over- or underdiagnosis. This chapter will explore the differential features between these two entities. The identification of underlying acute lung injury is an important factor to support reactive type II pneumocyte hyperplasia. The proliferating type II pneumocytes typically have uniformly enlarged nonoverlapping vesicular nuclei with a regular and smooth nuclear membrane and a single prominent nucleolus. The N/C ratio and apical-basal polarity are maintained. The overall configuration of the alveoli is preserved. The septa are usually slightly thickened by reactive stromal cells including reactive endothelial cells and inflammatory cell infiltrates