retroperitoneal inflammatory myofibroblastic tumor (Monarch Disease Ontology term MONDO_0006398) is a multinodular intermediate fibroblastic neoplasm arising from the retroperitoneum. It is characterized by the presence of spindle-shaped fibroblasts and myofibroblasts, and a chronic inflammatory infiltrate composed of eosinophils, lymphocytes and plasma cells. Also known as: retroperitoneal space inflammatory myofibroblastic tumor, retroperitoneal space inflammatory myofibroblastic tumour.