IMRT delivery of preoperative, high dose radiotherapy to a large volume, with Simultaneous Integrated Boost (SIB) in retroperitoneal sarcomas: The Ottawa experience C. Nessim, K. Carty and S. El-Sayed ; University of Ottawa, Ottawa, ON, Canada, The Ottawa Hospital Cancer Centre, Ottawa, ON, Canada INTRODUCTION PURPOSE RESULTS RESULTS Complete resection is considered the standard treatment of Retroperitoneal Sarcoma (RPS) (typically en bloc resection of tumor + involved organs) The most important prognostic factors for recurrence and overall survival are tumor grade, histologic subtype, site, use of radiation and R0 resection Achieving negative surgical margins (R0) remains a challenge in this context Since most recurrences are local, there is an increasing interest in the role of RT, partly based on randomized data from extremity and trunk sarcoma and partly based on retrospective institutional series , in an attempt to reduce local recurrence. Pre-op RT has been proposed as superior to post-op RT due to good tumor volume definition, displacement of normal viscera by tumor, smaller treatment fields, and potential radio-biologic advantage of having normal vasculature/oxygenation in place The benefit of RT remains debatable. RT may be hampered by the need to treat very large volumes while trying to limit the dose to sensitive organs. In most studies, the dose of radiation used has been very limited in order to limit the toxicity. The purpose of this study was to assess the feasibility of large volume treatment to tolerance dose of 45 Gy while boosting the GTV to a higher dose of 50-55 Gy using SIB with TOMO IMRT delivery . 23 patient s were treated. None developed any significant acute toxicity necessitating stopping radiotherapy. Grade 1 and 2 nausea = 60% Grade 1 and 2 Diarrhea.= 30% Tiredness = 80% No severe late effects have been reported. Despite the large volume treatment, surgical resections were successful in all but 3 patients. 2 refused surgery and 1 developed bone mets 2 patients required prolonged hospitalization after surgery No Immediate post-operative Mortality. METHODS Newly diagnosed patients with intermediate or high grade RPS Resectable tumour and no evidence of distant metastases. No Prior radiotherapy., ECOG 0, 1 or 2 All patients were treated with megavoltage radiation with IMRT delivery. Min dose = 45 Gy was delivered to full volume with a 5 mm margin to the PTV. Whenever possible an SIB was delivered to the GTV or the high grade areas to a dose of 50-55 GY all in 25 Daily Fractions. Surgery followed 4-6 weeks later. CONCLUSION IMRT delivery has facilitated the treatment of large retroperitoneal sarcomas with focused higher doses and acceptable toxicity. It remains to be seen whether this translates into long term Local control We await the results of the STRASS EORTC trial. RESULTS .