Pleural multicystic mesothelial proliferation. The so-called multicystic mesothelioma

We report on the clinical and pathological features of a hitherto unrecognized multicystic and multifocal mesothelial lesion arising in the pleural cavity of a 37-year-old Caucasian woman. The lesions consisted of clusters of thin-walled cysts separated by connective tissue and lined by a single layer of flattened and cuboidal mesothelium. Mucin stains, immunohistochemistry, and electron microscopy were consistent with a mesothelial origin. The pathological features are identical to those of the previously reported multicystic mesotheliomas of the peritoneum. Although these multicystic peritoneal mesothelial lesions have been regarded as neoplasms, absent stromal extension, lack of mitotic activity, and (in this case) continuity with morphologically normal surrounding mesothelium are suggestive of a reactive process. The term "multicystic mesothelial proliferation" may therefore be more appropriate. Because these lesions may be detected as discrete pleural based masses on chest radiograph and CT scan, they may be submitted for frozen section during operative resection. It is therefore important to be aware of their existence, morphology, and differential diagnosis.