Encapsulated type II pneumocyte adenoma: a case report and review of the literature.

An unusual benign lung neoplasm, a papillary adenoma of type II pneumocytes, was resected from a 26-year-old man who showed no clinical symptoms. The tumor was 2.0 cm in diameter and was localized in the subpleural region of S7 of the right lung; the cut surface showed a spherical medullary mass encapsulated by a thin layer of connective tissue. Histologically, there were cuboidal to columnar epithelial cells with a little nuclear atypia showing a monotonous papillary pattern with a delicate stroma in most parts of the tumor. There was neither capsular invasion nor metastasis of tumor cells. Nuclear DNA analysis of the tumor cells showed a diploid pattern and a low S-phase fraction. The immunohistochemical study revealed that most tumor cells contained a large amount of surfactant apoprotein in the cytoplasm. Osmiophilic lamellar bodies characteristic of type II pneumocytes were frequently found by electron microscopy. These findings indicate that this was a benign adenoma of the lung arising from type II pneumocytes.