[en] Purpose: A previous study of internal organ motion, in which a group of twelve patients with prostate cancer received CT scans on different days, showed variations in the position of the prostate and seminal vesicles, with larger variations observed in the latter. In addition, prostate and seminal vesicle positions were observed to be significantly correlated with changes in bladder volume. For most of the patients in the study the amount of bladder filling prior to scanning was not controlled, and large variations in bladder volume were observed for some patients. In light of these results, a second study in a larger group of patients has been undertaken, to examine the variability in the position of the prostate and nearby organs when the amount of bladder filling is controlled. Materials and Methods: Patients underwent four serial CT scans, consisting of an initial planning scan and a subsequent scan at the beginning, middle and end of the treatment course. Each scan was performed under the same conditions as for treatment, with the patient scanned in the prone position with an immobilization device. In order to minimize the variability in rectal and bladder filling, no bowel contrast was used and patients were asked to void immediately prior to each scan. Contours of the prostatic capsule, seminal vesicles, rectum, bladder and small bowel were drawn on the axial CT slices of each scan by a physician. The subsequent scans were matched to the planning scan by aligning the pelvic bones with a chamfer matching algorithm. Variability in target and nontarget organ positions and volumes were determined using the contours from the different scans. Results: An analysis has been performed on the data from the first sixteen patients and compared to the earlier study. The one standard deviation displacement of the prostate center of mass from the planning position was 0.19 cm and 0.32 cm in the lateral and anterior-posterior (AP) directions, respectively, which was comparable to the previous measurements (0.11 cm and 0.25 cm, respectively). For the seminal vesicles the standard deviation of the displacement of the center of mass was 0.20 cm laterally and 0.50 cm in the AP direction, the latter being somewhat less than in the previous study (0.14 cm and 0.72 cm, respectively). Significant correlation was observed between changes in rectal volume and prostate AP displacement (probability of no correlation p=0.025), as well as with seminal vesicle AP displacement (p<0.0001), which was consistent with previous observations. However, there was no significant correlation of bladder volume changes with AP displacement of the prostate (p=0.63) or the seminal vesicles (p=0.30), in contrast to the earlier study. This was likely due to the smaller variations in bladder volume that were observed: variation of bladder volume from the planning scan value was 43 cm³ (one standard deviation) with an average bladder volume of 90 cm³, compared to 112 cm³ and 182 cm³, respectively, in the earlier study. Further details of the analysis will be presented. Conclusion: Reduced variability of bladder filling indicates a reduced degree of seminal vesicle motion. However, the observed correlations with rectal volume imply that control of rectal filling is required to further reduce motion in the prostate and seminal vesicles