Influence of respiration-induced organ motion on dose distributions in treatments using enhanced dynamic wedges.

The mean velocity of respiration-induced organ motion in cranio-caudal direction is of the same magnitude as the velocity of the moving jaw during a treatment with an enhanced dynamic wedge. Therefore, if organ motion is present during collimator movement, the resulting dose distribution in wedge direction may differ from that obtained for the static case, i.e., without organ motion. The position as a function of time of the moving jaw has been derived from a log-file generated during each treatment. Parameters for the respiratory cycle and information about respiration-induced motion for organs in the upper abdomen were taken from the literature. Both movements were superimposed and the resulting monitor unit distribution has been calculated in the intrinsic coordinate system of the organ. The deviations from the static case have been studied as a function of wedge angle, amplitude of organ motion, respiratory rate, asymmetry of the respiratory cycle, beam energy, and the dose rate. If an amplitude of 30 mm and a respiratory rate of 10 min(-1) are assumed, the maximum deviation in monitor units is 2.5% for a 10 degees wedge, 7% for a 30 degrees wedge, and 16% for a 60 degrees wedge. Furthermore, a dose distribution for an organ undergoing respiration-induced motion has been generated and we found dose deviations of the same magnitude as calculated with the monitor unit distribution.