With the exception of well-differentiated liposarcoma, dedifferentiated leiomyosarcoma, solitary fibrous tumour, malignant peripheral nerve sheath and undifferentiated pleomorphic sarcoma, majority ≈70 histologic subtypes retroperitoneal sarcoma are defined as ‘ultra-rare’ sarcomas, with an incidence ≤1–5/1,000,000 persons/year. For most these ultra-rare RPS subtypes, diagnosis treatment follow...