Monarch Disease Ontology term MONDO_0006407 (sarcomatoid mesothelioma) is a diffuse malignant mesothelioma arising from the pleura and less often the peritoneum. It is characterized by the presence of spindle cells. Anaplastic morphologic features and multinucleated malignant cells may also be seen. Also known as: malignant fibrous mesothelioma, malignant fibrous mesothelioma (morphologic abnormality), sarcomatoid mesothelioma (morphologic abnormality), spindled mesothelioma.