Comparing Supine Versus Prone Positioning and Contouring Methods in Patients Undergoing Preoperative Chemoradiation for Rectal Cancer

0.002; 0.42, p<.0001; 0.29, p<.0001; -0.26, p Z .0004), but not BV. The distance between the bladder and rectum, d_(B2-AR2), was correlated with BV (r Z -0.27, p Z .0003) and RV (r Z -0.39, p Z <.0001). Multiple linear regression modeling of d_(B2-AR2) (when d_(B2-AR2)< Z 5 cm) can be predicted by BV, RV, and prior pelvic surgery: O(d_(B2AR2) (cm)) Z 1.89 0.001 x bladder volume(cc) 0.005 x rectum volume(cc) 0.035 x surgery (surgery Z 1 for prior prostatectomy or hysterectomy, surgery Z 0 otherwise). Age was found to have no predictive effect on d_(B2-AR2). Conclusions: The anterior rectum position is influenced by rectal volume, but not by bladder volume. Increasing bladder volume or rectal volume and prior prostatectomy or hysterectomy can reduce the distance between the two organs. These findings suggest that an additional margin on the rectum to account for bladder filling during rectal cancer radiation is not necessary. Author Disclosure: A.R. Bhirud: None. C. Lin: None.