Mesothelioma with clear cell features: an ultrastructural and immunohistochemical study of 20 cases

Summary

Mesotheliomas with clear cell morphology are rare and only a few individual case reports have been documented in the literature. The author reports a series of 20 epithelioid mesotheliomas with clear features, 17 of which originated in the pleura and 3 in the peritoneum. Eighteen of the patients were men and 2 were women. Twelve patients had a history of asbestos exposure. Electron microscopy and special histochemical stains demonstrated that the cytoplasmic clearing seen in hematoxylin and eosin–stained sections resulted from multiple factors that can occur either singly or in combination. The most frequent cause of the cytoplasmic clearing was the accumulation of large amounts of intracytoplasmic glycogen. Another but somewhat less common factor was the accumulation of large amounts of lipid, which occurred alone or with glycogen. Other less common causes were marked mitochondrial swelling, the presence of numerous intracytoplasmic vesicles, and a large number of intracytoplasmic lumens. The value of immunohistochemistry in helping to distinguish epithelioid mesotheliomas from some carcinomas with clear cell morphology is emphasized. In addition, it was determined that because electron microscopy was decisive in establishing the cause of the cytoplasmic clearing in most of the cases, tissue for electron microscopy should routinely be procured for ultrastructural studies.

Introduction

A well-known characteristic of tumors derived from the serosal membranes is their ability to grow in a wide variety of histologic patterns. Based on their light microscopic appearance, mesotheliomas have been subdivided into epithelioid, sarcomatoid, mixed epithelioid and sarcomatoid (biphasic), and desmoplastic subtypes. Although epithelioid mesotheliomas most frequently exhibit a tubulopapillary, tubular, or solid pattern, on rare occasions they may present other histologic patterns, including deciduoid, pleomorphic, small cell, signet-ring, and clear cell [1], [2], [3], [4]. Because general pathologists are less aware of the latter patterns, diagnostic difficulties may be encountered, especially in biopsy specimens, because these unusual morphological variants of mesothelioma can be confused with a variety of other neoplastic conditions. Although the presence of a few scattered cells or small clusters of cells exhibiting clear cell cytoplasm is not a rare finding in mesotheliomas, epithelioid mesotheliomas predominantly composed of cells having clear or finely vacuolated cytoplasm are uncommon and only a few individual case reports have been published [4], [5], [6]. The purpose of this study is to report a series of 20 mesotheliomas presenting clear cell morphology, investigate the causes of the clearing, discuss the differential diagnosis of these tumors, and make pathologists more aware of the existence of this variant of epithelioid mesothelioma.