Patent Number: 
Section: claims

1. A particle therapy apparatus comprising:a stationary 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 first or 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 including a first dipole magnet and a last dipole magnet and configured to bend the received particle beam along a beam path to a target,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; andan energy spread limiting device, installed at the first position, to limit 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 energy spread limiting device includes a momentum analyzing slit. 3. The particle therapy apparatus according to claim 1, wherein the energy spread limiting device includes a momentum analyzing aperture. 4. The particle therapy apparatus according to claim 1, wherein the energy spread limiting device includes a momentum analyzing collimator. 5. The particle therapy apparatus according to claim 1, wherein the particle beam generator is a cyclotron. 6. The particle therapy apparatus according to claim 1, wherein the first energy of the particle beam is between 230 MeV and 250 MeV. 7. 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. 8. The particle therapy apparatus according to claim 1, wherein the rotatable gantry is configured to rotate at least 180 degrees. 9. The particle therapy apparatus according to claim 1, wherein:the nominal dispersion is a transversal displacement of a particle having a 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. 10. The particle therapy apparatus according to claim 1, where the nominal dispersion at the first position is between 1 cm and 3 cm and the nominal beam size at the first position is between 0.2 cm and 1 cm. 11. The particle therapy apparatus according to claim 1, wherein the beam optic system further comprises a plurality of quadrupole magnets configured to perform at least one of focusing or defocusing the particle beam and wherein the first position is located after at least one quadrupole magnet of the plurality of quadrupole magnets. 12. The particle therapy apparatus according to claim 11, further comprising a controller configured to set the magnetic fields of at least one of one or more of the plurality of dipole magnets or of one or more of the quadrupole 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. 13. The particle therapy apparatus according to claim 12, wherein the energy spread limiting device has an opening through which at least a portion of the beam passes, and wherein the controller is configured to control the energy spread limiting device based, at least in part, on a calibration curve defining the opening of the energy spread limiting device as a function of an energy spread of the particle beam at an input of the energy spread limiting device. 14. A particle therapy apparatus comprising:a stationary particle beam generator configured to output a particle beam having a first energy;an energy degrader configured for receiving the particle beam having the first energy and 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 first or 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 bend the particle beam having the second energy along a beam path to a target, andan energy spread limiting structure located in the beam path between the first dipole magnet and the last dipole magnet, 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 to a selected maximum energy spread. 15. The particle therapy apparatus according to claim 14, wherein:the nominal dispersion is a transversal displacement of a particle having a 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. 16. The particle therapy apparatus according to claim 14, wherein the energy spread limiting structure includes a momentum analyzing slit. 17. The particle therapy apparatus according to claim 14, wherein the energy spread limiting structure includes a momentum analyzing aperture. 18. The particle therapy apparatus according to claim 14, wherein the energy spread limiting structure includes a momentum analyzing collimator. 19. The particle therapy apparatus according to claim 14, wherein the energy spread limiting structure is part of the gantry. 20. The particle therapy apparatus according to claim 14, wherein the particle beam generator is a cyclotron. 21. The particle therapy apparatus according to claim 14, wherein the first energy of the particle beam is between 230 MeV and 250 MeV. 22. The particle therapy apparatus according to claim 14, wherein the particle beam is a proton beam, and the second energy is between 70 MeV and 250 MeV. 23. The particle therapy apparatus according to claim 14, wherein the rotatable gantry is configured to rotate at least 180 degrees. 24. The particle therapy apparatus according to claim 14, further comprising a controller configured to set the magnetic fields of one or more of the plurality of dipole magnets. 25. The particle therapy apparatus according to claim 24, wherein the controller is configured to adjust magnetic strengths of one or more of the plurality of dipole magnets such that the particle beam having the second energy has the nominal dispersion larger than the nominal beam size at the first position. 26. The particle therapy apparatus according to claim 14, wherein the nominal dispersion at the first position is between 1 cm and 3 cm and wherein the nominal beam size at the first position is between 0.2 cm and 1 cm. 27. A particle therapy apparatus comprising:a stationary 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 first or 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 including a first dipole magnet and a last dipole magnet and configured to bend the received particle beam along a beam path to a target,wherein the beam optic system is configured to have a first position along the beam path where a nominal dispersion of the beam having the second energy is larger than a nominal beam size of the beam having the second energy, and wherein the first position is located downstream of the first dipole magnet and upstream of the last dipole magnet; andan energy spread limiting device, installed at the first position, to limit the energy spread of the particle beam having the second energy to a selected maximum energy spread.