This research study aims at defining 1) how retroperitoneal sarcomas change over the course of radiotherapy and 2) how radiotherapy affects your well-being. While the investigators know that radiotherapy before surgery is safe and effective, the amount of tumor motion and size change during radiotherapy is unknown. There is also very little information that describes the side-effects of radiotherapy in the treatment of this disease.

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.

Inclusion Criteria:

• Pathologically (histologically or cytologically) proven diagnosis of sarcoma (except for GIST, Ewing's sarcoma and rhabdomyosarcoma) within the retroperitoneum. These include patients with locally recurrent disease
• Planned to receive pre-operative radiotherapy
• Will undergo a planning CT
• Karnofsky Performance Status of ≥ 70 within 28 days prior to study entry
• No systemic chemotherapy given prior to pre-operative radiotherapy
• Able to provide written, informed consent
• Women of childbearing potential and men who are sexually active must practice adequate contraception.

Exclusion Criteria:

• Tumor pathology of GIST, Ewing's sarcoma or rhabdomyosarcoma
• Metastatic disease to the retroperitoneum from a primary sarcoma not originating from the retroperitoneum
• Systemic chemotherapy given prior to pre-operative radiotherapy.
• Intent on giving chemotherapy ≤ 12 months from the completion of radiotherapy
• Inability to undergo a 4D-CT simulation
• KPS < 70
• Unable to provide informed consent
• Pregnant women