Nodular histiocytic/mesothelial hyperplasia: a lesion potentially mistaken for a neoplasm in transbronchial biopsy

This report describes two examples of nodular histiocytic/ mesothelial hyperplasia as seen in transbronchial biopsy that initially led to serious consideration of neuroendocrine neoplasm or meningioma. The biopsies showed nodular collections of cohesive polygonal or round cells with ovoid or deeply grooved nuclei and a moderate amount of finely granular cytoplasm. Nuclear pleomorphism was mild. Immunohistochemical studies showed few cells staining for cytokeratin and the mesothelial marker HBME-1, whereas most cells were decorated by the histiocytic marker PG-M1 (CD68). This lesion appears to be identical to nodular mesothelial hyperplasia as described in hernia sacs and mesothelial/monocytic incidental cardiac excrescences, and we propose modifying the designation to "nodular histiocytic/mesothelial hyperplasia" to take into account the marked predominance of histiocytes over mesothelial cells. The clues to recognition of the true nature of the lesion are clinicopathologic correlation and identification of strips of low cuboidal (mesothelial) cells in the vicinity, and the diagnosis can be further confirmed by immunohistochemical staining. Nodular histiocytic/mesothelial hyperplasia probably results from irritation to the mesothelial lining by various causes leading to focal aggregation of histiocytes within retraction pockets or crevices of the serosal cavity.