Variants for a Riesenrad Ion Gantry

Hadrontherapy is widely believed to improve cancer treatment. The Bragg-peak effect of hadrons (protons and ions) makes them suitable for high precision and conformal scanning of tumours. To maximise the benefit it should be possible to deliver the particle beam from any direction in space towards the patient. A machine capable of performing such a task is called a medical gantry. So far only proton gantries have been built. The increased (magnetic) beam rigidity of say carbon ions yields considerable structural difficulties and has so far prevented realisation of an ion gantry. The structure would have to support large and very heavy bending magnets (50 t and more) and nevertheless deliver the beam with a sub-millimetre precision onto a patient in a supine position. The promising concept of a Riesenrad Gantry is suited to overcome this deadlock. The basic idea is to deflect the ion beam with a single 90° dipole, which rotates around the incoming beam axis, and direct it towards the eccentrically positioned patient cabin. Inside the (rotating) cabin similar conditions as in a classical isocentric treatment room prevail. The objective of this paper is to present and - to a certain degree - evaluate different versions of a Riesenrad gantry with clear focus on the mechanical performance. Two different ideas of how to achieve the specified accuracy are presented. Emphasis is also given to the decision making process leading to the designs.