Individually wide range of renal motion evaluated by four-dimensional computed tomography

Objectives: Assessment of physiologic renal motion in order to optimize abdominal intensity-modulated radiation therapy and stereotactic body radiation therapy. Methods and materials: Twenty patients with a median age of 47 years underwent computed tomography simulation and four-dimensional computed tomography acquisition. Thirty-nine kidneys were contoured during ten phases of respiration to estimate renal motion. Results: Kidney motion was not related to age (p = 0.42), sex (p = 0.28), height (p = 0.75), or body weight (p = 0.63). The average +/- standard deviation (SD) of movement of the center of gravity for all subjects was 11.1 +/- 4.8 mm in the cranio-caudal (CC) direction (range, 2.5-20.5 mm), 3.6 +/- 2.1 mm in the anterior-posterior (AP) direction (range, 0.6-8.0 mm), and 1.7 +/- 1.4 mm in the right-left (RL) direction (range, 0.4-5.9 mm). Renal motion strongly correlated with the respiratory phases (r > 0.97 and p < 0.01 in all three directions). Conclusions: Renal motion was independent of age, sex, height, or body weight. Renal motion in all directions was strongly respiration dependent, but motion in the cranio-caudal direction showed wide individual variation. In a clinical setting, it will be necessary to evaluate renal respiratory motion separately in each individual.