A Four-Dimensional Computed Tomography Analysis of Multiorgan Abdominal Motion

Purpose To characterize and quantify multiorgan respiration-induced motion in the abdomen in liver and pancreatic cancer patients. Methods and Materials Four-dimensional computed tomography scans were acquired for 18 patients treated for abdominal tumors. Contours of multiple abdominal organs were drawn by the radiation oncologist at one respiratory phase; these contours were propagated to other respiratory phases by deformable registration. Three-dimensional organ models were generated from the resulting contours at each phase. Motions of the bounding box and center of mass were extracted and analyzed for the clinical target volume and organs at risk. Results On average, the center of mass motion for liver clinical target volumes was 9.7 mm (SD 5 mm) in the superior–inferior direction, with a range of 3 to 18 mm; for pancreatic tumors, the average was 5 mm (SD 1 mm) m with a range of 3 to 7 mm. Abdominal organs move in unison, but with varying amplitudes. Gating near exhale (T40–T60) reduces the range of motion by a factor of ∼10. Conclusion We have used deformable registration to calculate the trajectories of abdominal organs in four dimensions, based on center of mass and bounding box motion metrics. Our results are compared with previously reported studies. Possible reasons for differences are discussed.