Nodular histiocytic/mesothelial hyperplasia: a potential pitfall

We present five cases of nodular histiocytic/mesothelial hyperplasia (two peritoneal, two pulmonary, and one pericardial) with identical microscopic features. All the lesions were biphasic and composed of cohesive monotonous epithelioid clusters of polygonal or oval cells with round or deeply grooved nuclei in association with darker cuboidal cells. Because of the increased cellularity and monotonous histologic pattern with some degree of cytologic atypia, neoplastic processes were seriously considered in the differential diagnoses. The majority of the cells marked as histiocytes by immunostain. A few scattered individual cells or small epithelial cell clusters were confirmed by calretinin stain to be mesothelial cells. The histologic patterns of the current lesions, irrespective of the location, were identical to nodular histiocytic/mesothelial hyperplasia. Histiocytic proliferations can be erroneously confused with primary mesothelial lesions or neoplasms such as granulosa cell tumor, eosinophilic granuloma, chronic myelogenous leukemia, and carcinoma. The purpose of this article is to describe the clinicopathologic features of nodular histiocytic/mesothelial hyperplasia and help familiarize pathologists with this lesion to prevent an erroneous diagnosis, particularly when it occurs in locations where mesothelial cells are not normally present.