Monarch Disease Ontology entry MONDO_0003376 (mediastinum leiomyosarcoma) is an aggressive malignant smooth muscle neoplasm, arising from the mediastinum. It is characterized by a proliferation of neoplastic spindle cells. Also known as: leiomyosarcoma of mediastinum, leiomyosarcoma of the mediastinum, mediastinal leiomyosarcoma.