Preliminary Experience of Reducing Bowel Irradiation Volume in the Treatment of Rectal Cancer: Two Cases Study

Abstract

Purpose: The dosage of radiotherapy to the pelvis is always limited by the tolerance of small bowel. This study is to evaluate the dose volume histogram (DVH) of the irradiated bowel volume by prone position with belly board in comparison to treatment in supine position technique. Two patients with rectal cancer were analyzed. Materials and Methods: Two groups of sequential computerized tomography (CT) scans were performed with 8-mm slice thickness for each patient. One group of the scans was obtained for patient with a belly board in a prone position, and the other one was in a supine position for comparing the bowel irradiation volume with the previous scan. Hard copies of CT scan slices for both patients were produced. The contours of small bowel volumes were outlined on each CT slice hard copy and the information was then digitized and entered into treatment planning computer. The cumulative and the differential DVHs were performed for evaluating the bowel irradiation volume. Results: The cumulative DVH, which is used in assessing potential damage to normal tissue, shows that from 70% to 95% of the percentage dose in prone position has a 15% to 18% volume reduction compared to supine position. The differential DVH, which is useful in assessing uniformity of dose to tumors. shows that the irradiated bowel volume has been moved to a region of rower percentage dose. Conclusion: Displacement of bowel loops out of the true pelvis is done by using a belly board in the prone position for thin and obese patients instead of supine position to decrease the volume of the intestine under irradiation because of external compression. The pelvic board and a half-body cast can be applied to immobilize patient for precise positioning and repositioning throughout the entire treatment.