Patent Number: 
Section: claims

1. A radiation therapy device, comprising: a radiation source positioned to direct a beam along a beam path toward a treatment area;  a treatment head containing a first collimator controllable to selectively collimate said beam, the treatment head including an accessory tray mounted between said first collimator and said treatment area; and  a second collimator removably mounted on said accessory tray and controllable to selectively collimate said beam, the second collimator including drive electronics removably mounted on an exterior of said accessory tray and a plurality of leaves removably mounted on said accessory tray and positionable by said drive electronics to move across said beam path. 2. The radiation therapy device of  claim 1 , wherein said first and said second collimators are controllable to selectively collimate said beam. claim 1 3. The radiation therapy device of  claim 1 , further comprising a first collimator drive operable to selectively position individual leafs of said first collimator. claim 1 4. The radiation therapy device of  claim 1 , further comprising: claim 1 a helium-filled container positioned along said beam path between said beam source and said second collimator. 5. The radiation therapy device of  claim 1 , wherein said drive electronics of said second collimator positioned a distance from said beam path. claim 1 6. The radiation therapy device of  claim 1 , wherein said radiation source includes a photon radiation source and an electron radiation source. claim 1 7. The radiation therapy device of  claim 6 , wherein said first collimator is controllable to selectively collimate a photon beam generated by said photon radiation source. claim 6 8. The radiation therapy device of  claim 6 , wherein said second collimator is controllable to selectively collimate an electron beam generated by said electron radiation source. claim 6 9. The radiation therapy device of  claim 1 , further comprising a control unit coupled to said radiation source and to said first collimator and to said drive electronics of said second collimator to selectively deliver a prescribed dose of radiation to said treatment area. claim 1 10. The radiation therapy device of  claim 9 , wherein said control unit is operable to control said radiation source to generate a photon beam and to cause said drive electronics of said second collimator to position leaves of said second collimator away from said beam path to deliver a prescribed dose of photon radiation to said treatment area. claim 9 11. The radiation therapy device of  claim 9 , wherein said control unit is operable to control said radiation source to generate an electron beam and to cause said first collimator drive to position leaves of said first collimator away from said beam path to deliver a prescribed dose of electron radiation to said treatment area. claim 9 12. A radiation therapy system, comprising: a control unit;  a treatment head having an enclosed area and an accessory tray;  a photon radiation source, selectively operated by said control unit to generate a photon beam along a beam path from said treatment head toward a treatment zone;  an election radiation source, selectively operated by said control unit to generate an electron beam along said beam path from said treatment head toward said treatment zone;  a photon collimator, located between said photon radiation source and said treatment zone; and  an electron collimator, removably mounted on said accessory tray, said electron collimator selectively positioned by said control unit to collimate said electron beam, the electron collimator including drive electronics removably mounted on an exterior of said accessory tray and a plurality of leaves removably mounted on said accessory tray and positionable by said drive electronics to move across said beam path. 13. An electron collimator for use in collimating an electron beam in a radiation therapy device, the collimator comprising: drive electronics, removably mounted on an exterior of an accessory tray of said radiation therapy device; and  a plurality of leaves positionable by said drive electronics to move across a path of said electron beam, said plurality of leaves removably mounted on said accessory tray of said radiation therapy device. 14. A radiation therapy device, comprising: a radiation source positioned to selectively direct an electron beam and a photon beam along a beam path toward a treatment area;  a treatment head containing a first collimator controllable to selectively collimate said photon beam, and including an accessory tray positioned between the radiation source and the treatment area; and  a second collimator removably mounted on said accessory tray and controllable to selectively collimate said electron beam, the second collimator including drive electronics removably mounted on an exterior of said accessory tray and a plurality of leaves removably mounted on said accessory tray and positionable by said drive electronics to move across said beam path. 15. A radiation therapy device, comprising: a control unit;  a radiation source, controlled by said control unit to generate one of a photon beam and an electron beam along a beam path toward a treatment area;  a first collimator, positioned between said radiation source and said treatment area, said first collimator selectively positioned by said control unit to collimate said photon beam;  an accessory tray positioned between said first collimator and said treatment area; and  a second collimator, removably mounted on said accessory tray, said second collimator selectively positioned by said control unit to collimate said electron beam, the second collimator including drive electronics removably mounted on an exterior of said accessory tray and a plurality of leaves removably mounted on said accessory tray and positionable by said drive electronics to move across said beam path. 16. The radiation therapy device of  claim 15 , further comprising: claim 15 a container positioned along said beam path between said first and second collimators. 17. The radiation therapy device of  claim 16 , wherein said container is filled with helium. claim 16 18. A radiation therapy method, comprising: installing an external collimator on an accessory tray of a radiation therapy device such that drive electronics of said external collimator are mounted on an exterior of said accessory tray;  operating a radiation source to direct a beam from a treatment head along a beam path toward a treatment area;  selectively controlling an internal collimator to collimate said beam;  selectively controlling said external collimator to collimate said beam, said external collimator removably positioned between said internal collimator and said treatment area. 19. A radiation therapy method, comprising: installing an external collimator on an accessory tray of a radiation therapy device such that drive electronics of said external collimator are mounted on an exterior of said accessory tray:  selecting between an electron treatment beam and a photon treatment beam;  directing said selected beam from a radiation source along a beam path toward a treatment area;  selectively controlling a photon collimator to collimate said selected beam if said selected beam is said photon beam; and  selectively controlling said external collimator to collimate said selected beam if said selected beam is said electron beam, wherein said external collimator is positioned between said photon collimator and said treatment area.