All patients have biopsy proven soft tissue sarcoma and undergo preoperative radiotherapy. Dynamic contrast-enhancing MRI T1-weight DCE-MRI will cover the whole tumor and involved lymph nodes if indicated. DCE-MRI is obtained before radiation starts and 4 weeks after RT completes. Blood volume images are estimated by using general Toft model, by which the blood flow was corrected for vascular permeability and perfusion. These images are geometrically co-registered with post-Gd T1-weighted MR images and pre-op RT routine MRI images are fused with the treatment planning CT.