Particle therapy gantry with an energy degrader and an achromatic final bending system

Abstract

A movable gantry for delivery of a particle beam using beam scanning technique contains an inlet section for an accelerated particle beam having quadrupole magnets, first and second bending sections having dipole and quadrupole magnets for beam correction, a transfer section having quadrupole magnets for beam correction and a degrader and a last beam bending section having separate and/or combined dipole/quadrupole/higher order multipole magnets forming an achromatic section. All the magnets of the achromatic last bending section are located downstream of the degrader. Any dispersion in this achromatic last bending section is suppressed. A scanning section having two separate or one combined fast deflection magnets that deflect the beam at the iso-center in a direction perpendicular to the beam direction to perform lateral scanning is provided. A beam nozzle section is provided and has a beam nozzle.

Claims

1. A movable gantry for delivery of a particle beam using beam scanning technique, the movable gantry comprising: an inlet section for an accelerated particle beam and having a plurality of quadrupole magnets; a first bending section having a plurality of dipole and quadrupole magnets for beam correction; a transfer section having a plurality of said quadrupole magnets for the beam correction and a degrader; a last beam bending section having a plurality of separate and/or combined dipole/quadrupole/higher order multipole magnets forming an achromatic section, wherein all said magnets of said achromatic section disposed downstream of said degrader, any dispersion in said achromatic section being suppressed so that the dispersion will have a momentum acceptance of more than 5%; a scanning section having two separate or one combined fast deflection magnets that deflect the particle beam at an iso-center in a direction perpendicular to a beam direction to perform lateral scanning; and a beam nozzle section having a beam nozzle.

2. The gantry according to claim 1, further comprising a collimator or collimator system disposed downstream of said degrader.

3. The gantry according to claim 1, wherein said scanning section is disposed upstream or within or downstream of said last beam bending section.

4. The gantry according to claim 1, wherein said first bending section forms a further achromatic section or a combination of several achromatic sections.

5. The gantry according to claim 1, wherein the gantry is rotatable around a longitudinal axis.

6. The gantry according to claim 1, wherein the gantry is rotatable around a horizontal axis perpendicular to the direction of the particle beam entering the gantry.

7. The gantry according to claim 1, further comprising a second bending section having a plurality of said dipole and quadrupole magnets; wherein said first and second bending sections has further magnets for the beam correction; wherein said transfer section has further magnets for the beam correction; and wherein said nozzle section further has beam handling equipment selected from the group consisting of further beam degrading or modifying elements and beam quality related beam verifying elements.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

(1) Preferred embodiments of the present invention are described hereinafter with reference to the attached drawings which depict in:

(2) FIG. 1 schematically the layout of the ProNova superconducting SC360 Gantry;

(3) FIG. 2 schematically the layout of a NIRS superconducting carbon ion gantry;

(4) FIG. 3 schematically the layout of scanner magnets being located upstream (left) or downstream (right) of the final bending magnet;

(5) FIG. 4 schematically illustration of PSI Gantry 2 (topFIG. 4A)and an example of a gantry with local achromaticity in each bending section and a degrader (bottomFIG. 4B) according to the present invention with their dimensions;

(6) FIG. 5 the transport simulation result of the beam optics of the gantry shown in FIG. 4, bottom part; and

(7) FIG. 6 the beam optics of the gantry after the collimation point Col2 for the following three cases: Top: magnets set at the value corresponding to the beam momentum, Middle: magnets set at 10% more than the value corresponding to the beam momentum and Bottom: magnets set at 10% more than the value corresponding to the beam momentum and with a beam divergence of 21 mrad at Col2.

DESCRIPTION OF THE INVENTION

(8) A possible option for a beam optics design of a gantry based on the requirements specified above has been developed and is described in more detail in the following.

(9) The layout of PSI's Gantry 2 has been used as a template in the design of this version of the gantry discussed here. FIG. 4 shows the layout of a gantry 2 according to the present invention (bottom) compared with PSI Gantry 2 (top). In FIG. 4 for the PSI gantry 2, three dipole magnets D1, D2 and D3 and seven quadrupoles Q1 to Q7 and two kicker magnets K1, K2 are shown. In FIG. 4, bottom, for the new design of the gantry 2 quadrupole magnets Q1 to Q8 and combined function magnets (dipole and quadrupole) C1 to C11 and the scanning magnets K1 and K2 are provided. In both designs, there are two bending sections 8, 12 of 60 and one last bending section 16 of 90. However, in the gantry 2 the bending sections 8, 12, 16 comprise several subsequent combined function magnets C1 to C11 with overlapping dipole and quadrupole fields. Additionally, the design contains eight g quadrupoles Q1 to Q8 before and between the bending sections (8, 12). Further, a first collimator Col1 and a second collimator Col2 have been added as well as a degrader D which is disposed upstream of the second collimator Col2. The gantry 2 as a whole is rotatable around the z-axis as shown in FIG. 4b.

(10) Scanning is implemented upstream of the final last bending section 16, requiring a relatively large aperture of the final bend magnets C7 to C11. With the magnets C1 to C11 the size of the gantry 2 is approximately 3.0 m in radius and 8.5 m in length.

(11) Following our invention it is considered to mount the degrader D before the last bending section 16 in the design of the gantry 2 (see below). To obtain a good beam transport the beam should have a small diameter when entering the degrader D. The first collimator Col1 is disposed at the coupling point 6. The round collimator aperture of this first collimator Col1 at the coupling point 6 at the entrance of the gantry 2 is imaged to the second collimator Col2 being disposed downstream of the degrader D between the second bending section 12 and the third bending section 16. The (1) beam size at this second collimator Col2 is 1.25 mm1.25 mm. From this second collimator Col2 a point-to-point imaging is made to the iso-center, so that the beam spot size there is 2.5 mm2.5 mm (at 1) in first-order.

(12) As discussed above, most existing gantries are achromatic as a whole but usually the achromaticity is not restored within each individual bending section (global achromaticity). As a consequence the dispersion can become very large within the gantry. This limits the momentum acceptance of the globally achromatic system. In the example of the gantry design presented above, each bending section 8, 12, 16 is achromatic by itself (local achromaticity). The dispersion function never reaches a high value in that case. Using this feature, the gantry design presented here, has a momentum acceptance of >10%. This means that without a change of the currents in the superconducting magnets, a beam with a momentum deviation of up to 10% (corresponding to the energy deviation of almost 20%) can still pass through the aperture of the gantry magnets and the vacuum pipe.

(13) The advantage of the degrader D at this location is that the beam size is small, so that shifting (e.g. carbon) degrader plates into the beam trajectory, can be done within several milliseconds only. This would allow very fast energy changes.

(14) In the design presented here the beam forms a waist at the position of the second collimator Col2, which is designed such that the beam divergence is high by strong focusing, so that the degrader D will not increase the beam divergence too strongly.

(15) An alternative possibility in the design is another location of the scanning magnets.

(16) The scanning magnets could be positioned within or behind the last bending section 16. A possible advantage could be that a smaller aperture of the magnets in the last bending section 16 can be used, without reducing the momentum acceptance.

(17) The gantry 2 combines in a smart way existing beam handling methods enabling new options for the beam optics in a gantry. These include a strong dispersion suppression within each single bending section. The local dispersion suppression will keep the maximum value of dispersion low along the whole beam line of the gantry 2. In the present invention, this property is used to accept a very large energy spread to enable the transport of an energy modulated beam without adjusting the bending field. This enables a very fast beam energy modulation, which is an important advantage in proton therapy.