PAPILLARY ADENOMA OF TYPE-II PNEUMOCYTES MIGHT HAVE MALIGNANT POTENTIAL

Papillary adenoma of type II pneumocytes is a rare tumour. It is consi dered to be a benign neoplasm and is derived from immature cells in th e bronchioloalveolar epithelium, however, its biological nature has no t been elucidated. We report a case of an adenomatous tumour; a papill ary adenoma of type II: pneumocytes, which we regard as possessing mal ignant potential. Light microscopically, as well circumscribed, papill ary tumour of predominantly cuboidal cells resembling type II pneumocy tes was found, but Clara type and ciliated cells were also present. Im munohistochemically, the tumour cells reacted positively with antibodi es to surfactant apoproteins (A, B), carcinoembryonic antigen, cytochr ome P-450 1A1-2 and 2B1-2. Ultrastructurally, many osmiophilic lamella r bodies and electron-dense granules were demonstrated. Semi-serial se ctions revealed signs of transbronchial dissemination and vascular inv asion. Morphometry using 12-dimensional cluster analysis disclosed fea tures of the tumour cells which resembled those of pneumocyte type II adenocarcinoma. These findings suggest that the present case has some malignant characteristics and originates from immature bronchiolar or alveolar cells, with a potential to develop into both type II pneumocy te and Clara cell type adenocarcinomas.