A very rare malignant tumor with morphologic features similar to those of benign perineurioma of soft tissue along with hypercellularity, nuclear atypia, hyperchromasia, and a high mitotic rate. The disease is malignant perineurioma (MONDO_0016751, a Monarch Disease Ontology id). Also known as: malignant peripheral nerve sheath tumor with perineurial differentiation, malignant peripheral nerve sheath tumour with perineurial differentiation, perineurial malignant peripheral nerve sheath tumor, perineurial malignant peripheral nerve sheath tumour, perineurioma, malignant.