Patent Number: 
Section: claims

1. A particle therapy apparatus comprising:a particle beam generator configured to output a particle beam having a first energy;an energy degrader for reducing an energy of the particle beam from the first energy to a second energy, such that the energy spread of the particle beam output by the energy degrader is increased with respect to an energy spread of the particle beam output by the particle beam generator; anda rotatable gantry configured to receive the particle beam having the second energy in a direction substantially along a rotation axis of the rotatable gantry, the rotatable gantry including:a beam optic system having a plurality of dipole magnets configured for transporting the particle beam along a beam path to a target,wherein the beam optic system includes, with respect to the beam path, a first dipole magnet and a last dipole magnet,wherein the beam optic system is configured to have a first position along the beam path where a nominal dispersion at the first position is larger than a nominal beam size at the first position, and wherein the first position is located downstream of the first dipole magnet and upstream of the last dipole magnet; anda spread-limiting device, installed at the first position, to limit an energy spread of the particle beam having the second energy, such that the limited energy spread is decreased, with respect to the energy spread of the particle beam having the second energy, to a selected maximum energy spread. 2. The particle therapy apparatus according to claim 1, wherein the spread-limiting device includes a spread-limiting aperture. 3. The particle therapy apparatus according to claim 1, wherein the spread-limiting device includes a collimator. 4. The particle therapy apparatus according to claim 1, wherein the particle beam generator is a cyclotron. 5. The particle therapy apparatus according to claim 1, wherein the first energy of the particle beam is an energy of 250 MeV. 6. The particle therapy apparatus according to claim 1, wherein the particle beam is a proton beam, and the second energy is between 70 MeV and 250 MeV. 7. The particle therapy apparatus according to claim 1, wherein the rotatable gantry is configured to rotate at least 180 degrees. 8. The particle therapy apparatus according to claim 1, wherein a controller defines the settings of the plurality of dipole magnets of the beam optic system. 9. The particle therapy apparatus according to claim 8, wherein the controller is configured to adjust magnetic strengths of one or more of the plurality of magnets of the beam optic system such that the particle beam having the second energy has the nominal dispersion larger than the nominal beam size at the first position. 10. The particle therapy apparatus according to claim 9, wherein the spread-limiting device has an opening through which at least a portion of the beam passes through, and wherein the controller is configured to control the spread-limiting device based, at least in part, on a calibration curve defining the opening of the spread-limiting device as a function of a momentum of the particle beam. 11. The particle therapy apparatus according to claim 1, where the beam dispersion is between 1 cm and 3 cm and the beam size is between 0.2 cm and 1 cm at the first position. 12. The particle therapy apparatus according to claim 1, wherein the first dipole magnet has a bending angle of 45° and the last dipole magnet a has a bending angle of 135°. 13. The particle therapy apparatus according to claim 1, wherein:the nominal dispersion is a transversal displacement of a particle having momentum differing by 1% of an average momentum of all particles of the beam, andthe nominal beam size being is a one sigma beam size value of a mono-energetic particle beam having the average momentum. 14. A particle therapy apparatus comprising:a particle beam generator configured to output a particle beam having a first energy;an energy degrader for reducing an energy of the particle beam from the first energy to a second energy, such that an energy spread of the particle beam output by the energy degrader is increased with respect to an energy spread of the particle beam output by the particle beam generator; anda rotatable gantry configured to receive the particle beam having the second energy in a direction substantially along a rotation axis of the gantry, and wherein the gantry includes:a plurality of dipole magnets including a first dipole magnet and a last dipole magnet configured to transport the particle beam having the second energy along a beam path to a target, anda spread-limiting structure located in the beam path at a first position where a nominal dispersion of the particle beam having the second energy is larger than a nominal beam size of the particle beam having the second energy and having a spacing to limit an energy spread of the particle beam having the second energy, such that the limited energy spread is decreased, with respect to the energy spread of the particle beam having the second energy, to a selected maximum energy spread value,wherein the first position is located downstream of the first dipole magnet and upstream of the last dipole magnet. 15. The particle therapy apparatus according to claim 14, wherein the first dipole magnet has a bending angle of 45° and the last dipole magnet has a bending angle of 135°. 16. The particle therapy apparatus according to claim 14, wherein:the nominal dispersion is a transversal displacement of a particle having momentum differing by 1% of an average momentum of all particles of the beam, andthe nominal beam size is a one sigma beam size value of a mono-energetic particle beam having the average momentum. 17. The particle therapy apparatus according to claim 14, wherein the spread-limiting structure includes a collimator. 18. The particle therapy apparatus according to claim 14, wherein the spread-limiting structure is part of the gantry. 19. The particle therapy apparatus according to claim 14, wherein the particle beam generator is a cyclotron. 20. The particle therapy apparatus according to claim 14, wherein the first energy of the particle beam is an energy of 250 MeV. 21. The particle therapy apparatus according to claim 14, wherein the particle beam is a proton beam, and the first energy is between 70 MeV and 250 MeV. 22. The particle therapy apparatus according to claim 14, wherein the rotatable gantry is configured to rotate at least 180 degrees. 23. The particle therapy apparatus according to claim 14, wherein a controller defines the settings of the plurality of dipole magnets of the beam optic system. 24. The particle therapy apparatus according to claim 23, wherein the controller is configured to adjust magnetic strengths of one or more of the plurality of magnets of the beam optic system such that the particle beam having the second energy has the nominal dispersion larger than the nominal beam size at the first position. 25. The particle therapy apparatus according to claim 14, where the beam dispersion is between 1 cm and 3 cm and the beam size is between 0.2 cm and 1 cm at the first position.