External postoperative radiation therapy for retroperitoneal sarcoma is an example of treatment using large fields for complex shaped volumes of irradiation. Prescribed dose is limited by tolerance of adjacent organs at risk (OAR). From a recent case treated by conventional conformal radiotherapy (3D-CRT), we evaluate the benefit of five theoretical IMRT plans. Criteria used are calculated from DVH related to delineated PTV and OAR. IMRT should permit to enhance the prescribed dose without increasing dose in the OAR (especially residual kidney, spinal cord and small bowel). This theoretical study show the feasibility of a dose escalation from a treatment dose of 45 Gy delivered by 3D-CRT up to a planning dose of 54 Gy calculated by IMRT with: for the PTV: an improvement of the dose homogeneity about 5% (range 2-6%) and moreover the coverage factor (CF) about 13% (range 9-16%); for the OAR: an improvement of the protection factor (PF) about 20% (range 11-24%); and thus an improved conformity index (CI = CF x PF) about 25% (range 15-32%).