This research study is a Phase I clinical trial. Phase I clinical trials test the safety of an investigational intervention. Phase I studies also try to define the appropriate dose of the investigational therapy to use for further studies. "Investigational" means that the intervention is still being studied and that research doctors are trying to find out more about it. Retroperitoneal sarcomas are soft tissue tumors located at the far back of the abdomen. Typically, patients with retroperitoneal sarcomas either have surgery for the removal of their tumors alone, or have their tumors removed, followed by standard radiation therapy, or have pre-operative radiation followed by surgery. When conventional radiation therapy is delivered after surgery, it can damage normal tissue. In this study, you will undergo proton beam radiation therapy or IMRT before undergoing surgery for the removal of your tumor. Proton radiation and IMRT are FDA approved radiation delivery systems. Protons are tiny particles with positive charge that can be controlled to travel a certain distance and stop inside the body. In theory, this allows better control of where the radiation dose is delivered as compared to photons. Since proton radiation is more targeted, it may help to reduce unwanted side effects from radiation. In this study, a standard dose of radiation will be given to the majority of the tumor, while a simultaneously integrated boost of additional radiation will be given to certain areas of the tumor identified as higher risk. This means that a higher radiation dose will be given to the higher risk areas of the tumor. The purpose of this study is to determine the highest dose of radiation therapy with protons or IMRT that can be delivered safely in patients with retroperitoneal sarcomas and the effectiveness of proton beam radiation therapy as an intervention for patients with retroperitoneal sarcomas.