Gantry 1

Gantry 1 was brought into clinical service in 1996. It provides treatments of deep-seated tumors. Gantry 1 was the first facility worldwide that used the magnetically controlled spot-scanning technique. This works like an old TV set, delivering the dose distribution point by point by steering the beam with magnets. In this way it is possible to generate any desired dose distribution in the 3 dimensions, without the need of collimators or compensators. The construction of the scanning system allows the proton beam to be delivered in parallel lines no matter what steering angle is used. This makes the beam position in the patient independent of the distance from the proton exit window (“parallel scanning”).

Scanning is facilitated in one dimension with use of sweeper magnets. The full area of ± 10 cm can be covered in 60 msec. For fine-tuning of the proton range, Gantry 1 uses 40 range-shifter plates placed in the nozzle, made from polyethylene, with a water-equivalent thickness of 4.7 mm. These plates are introduced into the beam path by means of a pneumatic system. The time taken to place a plate into the beam path is 60 msec. The patient lies on a motorized couch, which, at a speed of 2 cm/s, is the slowest of the three scanning dimensions. The irradiation of a reference volume of 1 litre takes around 3 minutes with this system.

Two plane-parallel ionization chambers placed in the nozzle are used for monitoring the dose. The position of the beam and the beam width are monitored in 2 dimensions with a strip chamber (4 mm strips), also placed in the nozzle.