Patent Number: 
Section: claims

1. An apparatus for controlling a radiotherapy electron beam, the apparatus comprising:an electron beam generator configured to generate an electron beam;a plurality of magnets producing a plurality of magnetic fields configured to focus the electron beam to a focal spot; anda control system configured to alter one or more parameters of the plurality of magnets to move the focal spot from a first location to a second location, wherein the first location is located at a first depth within a target region and the second location is located at a second depth within the target region. 2. The apparatus of claim 1 wherein:the target region is below an epidermal surface of a subject; andthe first location or the second location is at a depth between 0 and 50 centimeters from the epidermal surface. 3. The apparatus of claim 1 wherein the electron beam has a energy of between 50 and 250 megaelectron volts (MeV). 4. The apparatus of claim 3 wherein the energy of the beam is not modulated when the focal spot is moved from the first location to the second location. 5. The apparatus of claim 1 wherein the plurality of magnets comprise a plurality of collinear multipole magnets. 6. The apparatus of claim 5 wherein the plurality of collinear multipole magnets comprises at least two collinear quadrupole magnets. 7. The apparatus of claim 5 wherein:the one or more parameters of the plurality of magnets comprises a separation distance between the plurality of collinear multipole magnets; andthe control system is configured to alter the separation distance between the plurality of collinear multipole magnets. 8. The apparatus of claim 1 wherein the plurality of magnets comprise an anterior lens magnet, a posterior reflective magnet, and a plurality of radial focal magnets. 9. The apparatus of claim 1 wherein:the plurality of magnets comprise electromagnets;the one or more parameters of the plurality of magnets comprises an electrical current through the electromagnets; andthe control system is configured to alter the electrical current through the electromagnets. 10. The apparatus of claim 9 wherein the electromagnets are iron-dominated or coil dominated superconducting electromagnets. 11. The apparatus of claim 1 wherein the plurality of magnetic fields are configured to focus the electron beam at a convergence angle of between 50 and 500 mrad. 12. The apparatus of claim 1 wherein the plurality of magnetic fields are configured to focus the electron beam at a convergence angle of between 200 and 400 mrad. 13. The apparatus of claim 1 wherein the control system comprises an algorithm to calculate the one or more parameters of the plurality of magnets. 14. The apparatus of claim 13 wherein the control system receives input from a treatment planning software program configured to calculate a dose distribution. 15. The apparatus of claim 1 wherein the focal spot comprises a maximum electron dose concentration. 16. A method of controlling a radiotherapy electron beam, the method comprising:generating an electron beam;directing the electron beam through a plurality of magnetic fields produced by a plurality of magnets;focusing the electron beam to a focal spot with the plurality of magnetic fields; andaltering one or more parameters of the plurality of magnets to move the focal spot from a first location to a second location wherein the first location is located at a first depth within a target region and the second location is located at a second depth within the target region. 17. An apparatus for controlling a radiotherapy electron beam, the apparatus comprising:an electron beam generator configured to generate an electron beam with a power between 50 megaelectron volts and 250 megaelectron volts;a plurality of magnets configured to focus the electron beam at a focal spot; anda control system configured move the focal spot from a first location at a first depth to a second location at a second depth, wherein the power of the electron beam is maintained at a consistent level when the focal spot is moved from the first location to the second location. 18. The apparatus of claim 17 wherein the control system is configured to alter one or more parameters of the plurality of magnets to move the focal spot from the first location to the second location. 19. The apparatus of claim 18 wherein the control system comprises an algorithm to calculate the one or more parameters of the plurality of magnets. 20. A method of controlling a radiotherapy electron beam, the method comprising:generating an electron beam having a power between 50 megaelectron volts and 250 megaelectron volts;focusing the electron beam to a focal spot with a plurality of magnets; andmoving the focal spot from a first location at a first depth to a second location at a second depth while maintaining the power of the electron beam at a consistent level.