Engineering design and study of the beam position accuracy in the "Riesenrad" ion gantry

Beams of carbon ions are particularly well suited for radiotherapy. Their physical properties allow the 3D-conformal tumour irradiation with a sub-millimetre precision, provided that the beam is delivered by a rotating gantry equipped with a pencil-beam scanning system. However, the expected size and weight of such a carbon-ion gantry together with the requirement to direct the beam to the patient with an extreme position accuracy has so far prevented its realisation and stimulated the search for alternative solutions. One of them, the "Riesenrad" ion gantry, is introduced in the present paper. In contrast to conventional isocentric gantries, the main bending magnet of the Riesenrad is placed on the axis of gantry rotation, hence minimising the moment of inertia of the mobile structure and maximising its rigidity. The treatment cabin is smoothly moved towards the desired treatment position by a system that is mechanically de-coupled from the gantry. The engineering design as well as some aspects of the beam transport design of the Riesenrad are presented. Special attention is paid to the assessment of the beam position accuracy in the gantry isocentre as a function of elastic deformations and possible random errors such as temperature fluctuations. The study permits the specification of misalignment tolerances for the beam transport elements of the ion gantry. The most critical alignment error-transversal quadrupole shift-must be lower than 0.1 mm. (31 refs).