neoplasm mesothelioma Benign or malignant neoplasm derived from the lining cells of the pleura and peritoneum, composed of spindle cells or fibrous tissue which may enclose glandlike spaces lined by cuboidal cells. Epithelioid, fibrous, cystis, papillary, and sarcomatoid variants. (M- 9050/3). MPATH:365 mouse_pathology.ontology