A malignant neoplasm that arises from the soft tissues of the mediastinum. The disease is Monarch Disease Ontology id MONDO_0037743 (mediastinal soft tissue cancer). Also known as: malignant mediastinal mesenchymal tumor, malignant mediastinal mesenchymal tumour, malignant mediastinal soft tissue neoplasm, malignant mediastinal soft tissue tumor, malignant mediastinal soft tissue tumour, malignant soft tissue neoplasm of mediastinum, malignant soft tissue neoplasm of the mediastinum, malignant soft tissue tumor of mediastinum.