Diffuse malignant mesothelioma--epithelial type

Tumor almost always coats the pleura diffusely. The malignant mesothelial cells may have an epithelial, sarcomatous, or mixed pattern.

Here, tumor is composed of nests and sheets of polygonal, epithelial cells. Nuclei have mild pleomorphism and small nucleoli.

Diffuse malignant mesothelioma--epithelial type

This tumor shows glands and papillary structures, resembling adenocarcinoma.

Diffuse malignant mesothelioma--sarcomatous type

This tumor is composed of spindled cells and some collagenous stroma. The nuclei have irregular shapes. Lymphocytes surround a vessel near the bottom. The differential diagnosis includes other sarcomas.

Diffuse malignant mesothelioma--sarcomatous type

Besides cellular atypia, a feature that distinguishes malignant mesothelioma from reactive mesothelial proliferation is the presence of bland necrosis. Note that the cells between the arrows show fading of their nuclei (karyolysis).

Immunohistochemical results in malignant mesothelioma: For epithelial tumors, positive stains for carcinoma--PASD, CEA, B72.3, and CD15--exclude mesothelioma. For sarcomatous tumors, a positive stain for keratin supports a diagnosis of mesothelioma. More recently, the calretinin stain has been employed as a marker for mesothelial cells, either epithelial or sarcomatous. Usually several stains are employed in a particular case.

Ultrastructural features also help to distinguish carcinoma from mesothelioma. Carcinomas typically have few, short, non-branched microvilli. Epithelial mesotheliomas have abundant, long, branching microvilli (length to width ratio greater than 10-15/1) as shown in the photo. Note branch at arrow.

See also Case 12