A Feasibility Study to Compare the Benefits of Endoscopic Tumor Clip Placement and 3T-MRI Simulation for Accurate Target Volume Definition for Rectal Cancer Patients Undergoing Pre-operative Chemoradiation

This study is looking for a better way to define rectal tumors undergoing pre-operative radiation. MRI scanning generally results in more clear definition of the tumor that CT scanning. Insertion of radio opaque clips in the tumor site could also help to visualize the tumor better for radiotherapy treatment planning. The investigators believe both techniques would help the physician to define to cancer better than using CT scan alone. The investigators also will compare both techniques to define the better way to define the tumor.