Soft-tissue sarcomas (STS) that arise from the retroperitoneum are rare malignancies that are anatomical located deep within the abdominal area and thus pose challenges to surgical and radiotherapeutic management of the patient. As a result, the local control and overall survival for patients with retroperitoneal sarcomas (RPS) are worse than STS from the extremities. Current treatment strategy involves pre-operative radiotherapy followed by surgery. Use of intensity-modulated radiotherapy (IMRT) in RPS had allowed for more conformal treatments with the aim of sparing normal tissues from high doses of irradiation. Yet the accuracy and coverage of IMRT depend highly on target motion, and little is known about the motion of RPS during the course of radiotherapy. As well, RPS are commonly in close proximity to sensitive organs for which the long-term toxicity and effect on quality of life secondary to radiation is unknown. The current study seeks to evaluate the extent of tumor motion during radiotherapy and the impact of radiotherapy to patient toxicity and quality of life. At the conclusion of this study, our results will hopefully identify the optimum PTV, the importance of different normal tissues and their dose-volume constraints, the role of image guidance, and the potential for dose escalation in the treatment of RPS.