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 patient's 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.

Rectal cancer accounts for 5% of all cancers diagnosed in Canada for both men and women between 1995 and 2004 (Public Health agency of Canada _ Health 2004). Preoperative radiotherapy (RT), with or without chemotherapy, is an accepted treatment for patients with locally advanced rectal carcinoma followed by surgical resection (Bosset and Horiot 2001; Sauer, Fietkau et al. 2003; Birgisson, Talback et al. 2005). The current standard treatment at Princess Margaret Hospital (PMH) is preoperative pelvic radiation therapy to a total dose of 50 Gy/25 fractions/5 weeks. The total radiation dose is reduced to 45 Gy/25 fractions/5 weeks when there is a concern about the volume of small bowel within the treatment volume. Radiation therapy is delivered with 5-fluorouracil (5-FU), 225mg/m2/24h, by protracted venous infusion (PVI) or oral capecitabine, 825 mg/m2 BID for 5 weeks, starting the first day and completing the day of last day of radiation therapy.

Inclusion Criteria:

• Any rectal cancer patient who is to receive a 5 week course of preoperative pelvic radiation therapy with or without concurrent chemotherapy
• Prone treatment position
• Age 18 years or older
• Informed consent

Exclusion Criteria:

• Inability to provide informed consent
• Inability to tolerate treatment in the prone position
• Pregnancy
• Patients with unilateral or bilateral metallic total hip replacements