Matrix Analysis and Design Study of Rotatorlike Gantry Optics

Modern ion-therapy facilities are commonly equipped with rotating gantries to achieve higher dose conformity to the tumor. The gantry is the terminating part of the beamline from the accelerator to a gantry treatment room. The gantry is mechanically rotated around the patient. In synchrotron-based facilities, the slowly extracted beams have different emittance patterns in the two transverse planes (such beams will be referred to as asymmetric beams). Several matching techniques were developed in the past to remove the angular dependence of the beam parameters at the gantry isocenter on the gantry rotation angle. Recently, a novel so-called rotatorlike gantry optics has been introduced. In this concept, all existing matching techniques are integrated into the gantry optics and the gantry nozzle. The underlying theoretical description of its working principle is presented in this paper. The description is based on the first-order matrix analysis of the transport of asymmetric beams in rotating ion-optical systems, in general. The results are formulated in terms of ion-optical constraints imposed on the fixed incoming beamline, and the gantry transfer matrix. Theoretical matrix analysis is followed by a design study applied to the MedAustron proton-gantry layout as a typical representative of an isocentric gantry equipped with a pencil-beam scanning system.