Exuberant fibroadenomatoid proliferation in a pulmonary mesenchymoma (hamartoma): report of a lesion mimicking a sclerosing pneumocytoma.

We report the cytologic and histopathologic features of an unusual peripheral primary lung tumor in a 61-year-old female. The central portion showed features of typical pulmonary chondroid hamartoma (mesenchymoma). Arising from the peripheral margin of this tumor, however, was an exuberant proliferation of papillary mesenchymal structures lined by type 2 pneumocates, mimicking papillary sclerosing hemangioma (pneumocytoma). This extreme example of fibroadenomatoid proliferation shares histologic features with other pulmonary neoplasms characterized by mesenchymal cores lined by type 2 pneumocytes. We propose that such lesions be unified under the term "alveolar mixed tumor".