Answer: Minute meningiothelial lesion (“minute chemodectoma”)

Histology: none provided

Discussion: Minute pulmonary meningothelial-like nodules (MPMNs) are usually incidental interstitial pulmonary nodules, although mass lesions have rarely been described. MPMNs share histologic, ultrastructural, and immunohistochemical staining features with meningiomas although the genotypic comparison suggests that MPMNs are different from meningioma (Am J Surg Path 2004; 28:207-214). The characteristic histologic features is a distinctly interstitial proliferation of bland cells with spindle nuclei and a whorled growth pattern quite reminiscent of meningioma. The lesions are located in the interlobular septa near veins. For years, these lesions were considered to be minute chemodectomas and, based on their location, were thought to have some chemosensing function in the lung. Ultrastructural and immunocytochemical studies confirm the logic relationship of these lesions to meningioma (Am J Surg Path 1988; 12:167-175). The lesions are strongly reactive for epithelial membrane antigen and are generally uniformly negative for cytokeratin, S-100 and actin immunostains. MPMNs may be solitary or multifocal and are not associated with any specific disease or process. The usual differential diagnosis lies between MPMN and carcinoid tumorlet. The interstitial location of MPMN (verus the endobronchiolar origin of carcinoid tumorlet) and the differences in immunocytochemical reactivity of these lesions should resolve the question.