Exuberant type 2 pneumocyte hyperplasia associated with spontaneous pneumothorax: secondary reactive change mimicking adenocarcinoma.

A wide variety of pulmonary and pleural histological changes is recognized in the setting of spontaneous pneumothorax. In this study, we describe a previously unreported lesion that was encountered in four males, 24-41 years of age. In addition to reactive eosinophilic pleuritis, subpleural emphysematous blebs, prominent eosinophilic exudate and lung atelectasis, the histology comprised exuberant type 2 pneumocyte hyperplasia, which was atypical enough to consider a diagnosis of adenocarcinoma in all four cases. Lung atelectasis and localized acute lung injury are factors likely responsible for this unusual histology, and along with the clinical history are important in recognizing the benign nature of this lesion. Awareness of this severe pneumocyte reaction in the setting of pneumothorax can help to prevent misdiagnosis as malignancy.