[Compensating Patient Displacement by Couch Movements in Patient Positioning System]

It is important to position patient accurately in radiation therapy which has a good dose concentration especially, in charged particle radiotherapy. Misalignment of patient to the center of the beam axis causes complication of normal tissue and low dose in the target volume due to the concentration. Hyogo Ion Medical Center has a patient positioning system which can position within lmm. The system consists of X-ray TV system to detect displacement of patient and treatment couch which have 6 axes (3 translations and 3 rotations) to move patient accurately. Since the couch has 3 rotation axes, it is very difficult to express the displacement with translations and rotations of patient by the couch movements. Therefore, expression of translation and rotation to compensate the patient displacement was derived. Test of the expression was done in the patient positioning system. X-ray images were taken before and after moving the couch, and the actual displacement and the calculated one for 6 axis were compared. The actual and the calculated agreed each other within lmm for translation and 0.1 deg for rotation.