Oblique gantry — an alternative solution for a beam delivery system for heavy-ion cancer therapy

Beams of protons and heavy ions have favorable physical and biological properties for their application in radiotherapy. Their advantages can be best exploited if the beam can be applied to the patient from any direction by a rotating gantry. The construction of a gantry is quite demanding for heavy ions due to the high magnetic rigidity of the therapy beams. In order to reduce the gantry size and weight, a novel gantry concept with an oblique 60° output beam is proposed. This concept allows a very compact gantry design even for the isocentric layout and normalconducting magnets. The overall gantry radius is 2.8 m. The gantry is equipped with a two-directional magnetic scanning system and an achromatic beam transport system. The scanning system is located upstream to the last gantry dipole and combines a parallel scanning mode in one direction with a low-angle scanning mode in the other direction in order to reduce the vertical gap of the last dipole. The beam transport system is designed with a high degree of ion-optical flexibility which is used to form a narrow pencil-like beam with adjustable spot-size in the gantry isocentre. The design principles and ion-optical properties of the gantry beam transport and scanning systems are discussed.