There is good preliminary research on the benefits of eating a healthy diet before during and after cancer treatment. There have been many articles on particular foods or food groups (like the super foods; immune enhancing) that have shown to be or seem to show a benefit in prevention of cancer and a few that suggest that they improve response to treatment.
There is not enough substantiated research out there to definitively say that this group of foods alone can change the response to sarcoma.
My recommendations despite the lack of evidence is to incorporate cruciferous vegetables like broccoli, cauliflower, high flavanoid containing foods like onions, garlic and high anti-oxidant foods like berries into a balanced diet that includes 5-9 servings from the fruit and vegetable group every day, limit animal fats and trans fats. Limit or avoid empty calorie foods and select whole grain breads, cereals and grain products. Include vegetable source proteins as an alternative like soy beans. For additional information on current research check out the web site of the American Institute for Cancer Research (AICR).