Alveolar adenoma: A histochemical, immunohistochemical, and ultrastructural analysis of 17 cases

Alveolar adenoma of lung is a rare benign neoplasm of uncertain histogenesi s. Its rarity hampers characterization of its epithelial and mesenchymal el ements. Clinical and histopathologic features of 17 alveolar adenomas were reviewed. Histochemistry was performed on 10 cases, ultrastructural analysi s on two, and immunohistochemistry on six cases for pneumocyte markers, thy roid transcription factor (TTF-1), surfactant protein markers pro-SP-B and proSP-C, and the Clara cell marker, CC10. Immunohistochemistry was performe d in nine cases for desmin, smooth muscle actin, muscle-specific actin, cyt okeratin, proliferating cell nuclear antigen (PCNA), factor VIII, and carci noembryonic antigen. The mean age was 53 years. Seven cases occurred in men , and nine occurred in women. The age and sex were not known for one patien t. The tumors were coin lesions on chest radiographs in asymptomatic patien ts except for one (cough). The mean size was 2.2 cm. The tumors were well d emarcated with multiple cystic spaces containing granular material. Mostly type 2 pneumocytes lined the cystic spaces with fewer type 1 cells and no C lara cells. This was confirmed by staining for TTF-1, pro-SP-B, and pro-SP- C and by ultrastructure. CC10 was negative in all cases. The stroma varied from prominent spindle cells with a myxoid matrix to thin alveolar septa. T he interstitial spindle cells resembled fibroblasts by immunohistochemistry and ultrastructure. Follow-up data available in five cases showed no recur rence at 2, 2, 5, 8, and 13 years. In summary, alveolar adenoma is a benign neoplasm consisting of an intimate admixture of alveolar epithelial and se ptal mesenchymal tissue. Most of the epithelial cells are type 2 pneumocyte s, and the interstitial stromal cells are fibroblasts or fibroblast-like ce lls. Recognition of its characteristic morphological appearance allows for its distinction from other benign lesions of the HUM. PATHOL 30:158-167. Th is is a US government work. There are no restrictions on its use.