Abstract
When using accelerator beams for cancer therapy, the three-dimensional freedom afforded by a gantry helps the treatment planner to spread out surface doses, avoid directions that intercept vital organs and irradiate a volume that is conformal with the tumour. The general preference is for an iso-centric gantry turning 360° in the vertical plane around the patient bed with sufficient space to be able to orientate the patient through 360° in the horizontal plane. For hadrontherapy, gantries are impressive structures of the order of 10 m in diameter and 100 ton in weight and to date only proton gantries have been demonstrated to operate satisfactorily. The increased magnetic rigidity of say carbon ions will make ion gantries more difficult and costly to build. For this reason, exo-centric gantries and, in particular the so-called `Riesenrad’ gantry with a single 90° bending magnet, merit further attention. The power consumption is reduced and the heavy magnets with their counterbalance weight are reduced and are kept close to the axis. The treatment room, which is lighter, is positioned at a larger radius, but only the patient bed requires careful alignment. An optics module called a `rotator’ is needed to match an incoming dispersion vector to the gantry in order to have an achromatic beam at the patient. A practical design is described that assumes the beam is derived from a slow-extraction scheme in a synchrotron and that the beam sizes are controlled by modules in the transfer line. Magnetic scanning is integrated into the gantry optics for both transverse directions.
Highlights
The quality of cancer treatment with accelerator beams is greatly improved by the use of a gantry that can deliver the beam to the patient from any wanted direction
The three-dimensional freedom afforded by the gantry allows the treatment planner to spread out surface doses, avoid directions that intercept vital organs and to irradiate a volume that is conformal with the tumour
A complete solution for a light-ion gantry, operating with a synchrotron using resonant slow extraction has been presented. This solution, known as the ‘Riesenrad’, is an exo-centric gantry that keeps the heavy magnetic equipment on, or close to, the axis while the patient is positioned in a light treatment room which can move at a larger radius
Summary
The quality of cancer treatment with accelerator beams is greatly improved by the use of a gantry that can deliver the beam to the patient from any wanted direction. In order to limit the overall diameter, the scanning magnets are best positioned upstream of the last dipole. This requires a large aperture magnet which creates the problem of supporting, with high precision, a large mass in an off-axis position. For this reason, exo-centric gantries and, in particular the so-called ‘Riesenrad’ gantry, merit further attention [3]. Patient couch on high-precision robot arm Treatment room. X Only the patient couch requires high precision alignment relative to the final dipole; the positioning of the treatment room is not critical. The design of the scanning system and the main bend is discussed
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