Purpose: To evaluate changes in preoperative and postoperative positions of structures used to define target volumes (i.e., pancreatic bed, porta hepatis local-regional lymph nodes) for postoperative irradiation of pancreatic malignancies as defined by abdominal computed tomographs. Methods and Materials: Eleven consecutive patients who had Whipple resection and postoperative irradiation for pancreatic cancer were evaluated. Preoperative and postoperative computed tomographs of each patient were evaluated for the position of the portal vein bifurcation and the origin of the celiac axis and superior mesenteric artery. The length along the x (medial-lateral position) and y (anterior-posterior position) axes was determined with calipers to the closest millimeter. Length along the z axis (cephalad-caudad position) was determined with the computed tomographic sectional interval between images. Statistical significance of the change in the structure's position along the x, y, or z axis between preoperative and postoperative computed tomographs was assessed with the paired t-test. Results: Evaluation of the preoperative and postoperative positions of the portal vein, celiac axis, and superior mesenteric artery along the x y and Z axes revealed a statistically significant change in the location of the portal vein and celiac axis postoperatively. The median change of the celiac axis in the anterior- posterior position was significant (p = 0.0047), but the mean change was only 2 mm and not considered clinically significant. The median change for the portal vein was 0.97 cm and 1.07 cm along the y and x axes, respectively, and was significant (p = 0.008 and p = 0.0001). The range in position change for the portal vein was 0.0 to 2.0 cm along the y axis and 0.4 to 1.9 along the x axis. The remaining mean changes in position along all axes for all the structures were less than 3 mm (not statistically significant). Conclusions: The mean position of the portal vein-porta hepatis after Whipple resection is approximately 1.0 cm medial and 1.0 cm posterior compared with its preoperative position. These data suggest that postoperative abdominal computed tomographs are useful in determining treatment volumes of nodal drainage basins after Whipple resection of pancreatic malignancies.