Fig. 2

The prostate follows the couch shifts. A net shift of the couch of 6.5 mm corrects the prostate position to zero. The couch moves at a speed of 9.7 mm per second. The amplitude of the couch shift amounts to 31.5 mm. Still, neither the speed nor the magnitude of the couch shift affects the intra-fraction motion of the prostate (prostate position relative to the couch). The amplitude of intra-fraction motion is less than 1 mm, and the correlation coefficient between relative prostate position and couch position is only −0.06