Design of an achromatic and uncoupled medical gantry for radiation therapy
We are presenting the layout and the optics of a beam line to be used as a medical gantry in radiation therapy. The optical properties of the gantry's beam line are such as to make the beam line achromatic and uncoupled. These two properties make the beam spot size, which is delivered and focused by the gantry, on the tumor of the patient, independent of the angular orientation of the gantry. In this paper we present the layout of the magnetic elements of the gantry, and also present the theoretical basis for the optics design of such a gantry. A medical gantry, as it is used in the radiation treatment of cancer patients, is the last part of the beam optical system, of the accelerator complex, which delivers and focuses the beam on the tumor. The curved line shown in figure 1 is a schematic diagram of a gantry which can rotate about a horizontal axis. The particle beam (green arrow in fig. 1) enters the gantry, and is guided by the gantry on the tumor (red spot in fig. 1). As the gantry rotates about the axis shown in figure 1, the beam exiting the gantry always lies on a plane normal to the rotation axis at the point of the icocenter. Thus the gantry facilitates the ability of the beam delivery system, to deliver the beam at the tumor, which is placed at the icocenter, from any angle on this vertical plane, which is normal to the rotation angle of the gantry as stated earlier. The gantry consists of dipoles and quadrupoles elements whose median symmetry plane lies on a plane which contains the rotation axis of the gantry. In this paper we define this plane as the 'plane of the gantry'. As the beam is transported along the axis of rotation of the gantry and before it enters the gantry, it is focused by 'normal' quadrupoles and experiences no linear beam coupling. Subsequently the beam enters the gantry, and is transported by the gantry to the delivery point which is the tumor. The transported beam at the tumor is still linearly uncoupled as long as the plane of the gantry lies either, on the horizontal, or the vertical plane. In these two orientations of the gantry the dipoles and the quadrupoles are 'normal' therefore the linear R-transport-matrix appears linearly uncoupled. As the gantry rotates about this horizontal axis, to irradiate the tumor at a different angle, the linear beam coupling is unavoidable since the coordinate system of the gantry's quadrupoles is rotated with respect to the coordinate system of the quadrupoles at the entrance of the gantry which are located along the axis of rotation. As a result, in this rotated coordinate system, the beam will be linearly coupled as it is transported by the gantry to the tumor and the beam size at the location of the tumor will vary in size depending on the orientation of the gantry. In this paper we present a method which makes the beam transport R-matrix linearly uncoupled, achromatic and independent of the orientation angle of the gantry as is proven in the following section.
- Research Organization:
- Brookhaven National Lab. (BNL), Upton, NY (United States). Relativistic Heavy Ion Collider (RHIC)
- Sponsoring Organization:
- DOE - Office Of Science
- DOE Contract Number:
- DE-AC02-98CH10886
- OSTI ID:
- 1013545
- Report Number(s):
- BNL-94131-2011-CP; R&D Project: KBCH139; 18054; KB0202011; TRN: US1102522
- Resource Relation:
- Conference: 2011 Particle Accelerator Conference (PAC'11); New York, NY; 20110328 through 20110401
- Country of Publication:
- United States
- Language:
- English
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