A primary or metastatic malignant neoplasm involving the retroperitoneum. The vast majority of cases are carcinomas, lymphomas, or sarcomas. The disease is retroperitoneal cancer (MONDO_0005941). Also known as: cancer of retroperitoneal space, malignant neoplasm of retroperitoneal space, malignant neoplasm of retroperitoneum and peritoneum, malignant retroperitoneal neoplasm, malignant retroperitoneal space neoplasm, malignant tumor of peritoneum and retroperitoneum, malignant tumour of peritoneum and retroperitoneum, retroperitoneal space cancer.