The papillary architecture is well-defined with papillae containing myxoid stroma covered by a single layer of flattened or cuboidal epithelioid cells. Foamy macrophages may be present within the cores. The nuclei are uniform and lack conspicuous nucleoli. Mitotic activity is not increased. Distinction from epithelioid malignant mesothelioma may be extremely difficult in small biopsy specimens. Presence of solid areas, multicentric or diffuse lesions, stromal invasion, and necrosis favor malignant mesothelioma. Correlation with intraoperative findings and radiologic findings may be helpful.