Abstract
The alveolar adenoma of the lung is a rare benign tumor characterized by a proliferation of both the alveolar epithelial cells and the mesenchymal septal cells. Immunohistochemically, the epithelial cells stain for cytokeratin (CK) AE1AE3, CK7, thyroid transcription factor 1 (TTF1), and surfactant apoprotein confirming the derivation by the type 2 pneumocytes. The stromal cells are negative for these markers but they show focally smooth muscle and muscle-specific actin positivity. We describe two cases that showed immunohistochemically a CD34 positivity of the mesenchymal septal cells. This aspect has been previously described in a two cases report, but not emphasized by the authors as a distinctive feature of the lesion. We consider this CD34 positivity as a marker of immaturity or stemness of the lesional septal spindle cells, that could be responsible of the different phenotypic and morphological profile of the interstitial cells, that could be, therefore, considered neoplastic and not reactive.
Highlights
Alveolar adenomas are rare benign peripheral lung tumors first described by Yousem and Hochholzer [1]
The cysts are lined with a single layer of epithelial cells, with most of them being cuboidal or “hob nailed” in appearance and eosinophilic, finely vacuolated, or foamy cytoplasm, consisting in type 2 pneumocytes as showed by immunohistochemical and ultrastructural studies [1, 5, 6]
The epithelial lining cells stained for CK AE1/3, CK7, epithelial membrane antigen (EMA), transcription factor 1 (TTF1), and surfactant apoprotein indicating a type II pneumocyte phenotype, while stromal cells were positive for vimentin, for CD34 (Figure 3(b)), more intensively in the subepithelial spindle cells and focally for muscle-specific actin
Summary
Alveolar adenomas are rare benign peripheral lung tumors first described by Yousem and Hochholzer [1]. A few have been reported since [2,3,4,5,6,7,8,9,10,11,12] This neoplasm presents as a solitary peripheral lesion discovered incidentally on chest radiographs. These tumours manifest as well-demarcated spongy nodules with average size of 1.9 cm, which may be located in any lobe beneath an intact pleura.
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