 This study is designed to investigate whether or not the use of a customized vacuum immobilization device CVID to immobilize rectal cancer patients undergoing preoperative pelvic radiation therapy improves the positional stability of patients during each radiation treatment. This question is more relevant now in the era of image-guided radiation therapy IGRT. Using state of the art IGRT technologies such as cone beam CT CBCT scanning, assessments of the accuracy of treatment delivery can be made during a course of radiation therapy allowing for patient set-up corrective strategies to be implemented. The use of CBCT scanning may minimize any potential benefit of of a CVID if the patients pretreatment position can be corrected prior to treatment and our current immobilization strategy results in a stable patient position. While it is seemingly intuitive to assume that the use of CVID results in better patient stability, there is actually little data to support this. 