Surgery With or Without Neoadjuvant Chemotherapy in High Risk RetroPeritoneal Sarcoma

Standard arm: * Large en-bloc curative-intent surgery within 4 weeks following randomization- Experimental arm Experimental arm: * 3 cycles of neoadjuvant chemotherapy starting within 2 weeks following randomization: * High grade LPS: ADM (doxorubicin) 75 mg/m2 (or the equivalent EpiADM 120 mg/m2) + ifosfamide 9 g/m3 Q3 weeks. * LMS: ADM 75 mg/m2 + DTIC (dacarbazine) 1 g/m2 Q3 weeks * re-assessment of operability * curative-intent surgery within 3-6 weeks of last cycle of chemotherapy