Patent Number: 
Section: claims

1. A method of determining a shape of a radiation beam at a target position of a radiotherapy system, the system comprising:a radiation source for projecting the radiation beam towards the target position along a beam axis; anda multi-leaf collimator disposed between the radiation source and the target position, the multi-leaf collimator comprising a defocused array of moveable leaves configured to be positioned to intersect and block a part of the radiation beam so as to define the shape of the radiation beam at the target position, each leaf having a plane extending through a plane perpendicular to the beam axis and a width extending in a direction perpendicular to the plane of the respective leaf, wherein at least a first leaf and a second leaf are aligned such that the plane of the first leaf converges with the plane of the second leaf at a line, wherein a point within the line is displaced laterally from the beam axis;the method comprising:determining a projected width, with respect to the radiation beam, for each leaf positioned to intersect the radiation beam, the projected width being greater than a width of the respective leaf, wherein the determined projected width for a leaf further from the beam axis is wider than the determined projected width for a leaf closer to the beam axis;determining the shape of the radiation beam at the target position using the projected width of at least one leaf;positioning the leaves of the multi-leaf collimator based on the determined shape of the radiation beam; andirradiating the target position with the beam. 2. The method as claimed in claim 1, wherein all of the leaves of the multi-leaf collimator have substantially the same width, the method comprising:determining a progressively wider projected width for a leaf as a distance between the leaf and the beam axis increases. 3. The method as claimed in claim 1, comprising:determining the position of an edge of the radiation beam at the target position using the projected width of at least one leaf. 4. The method as claimed in claim 1, wherein the projected width is determined as a distance, in the plane perpendicular to the beam axis and passing through the leaf, between (i) a lateral edge extremity of the leaf furthest from the beam axis in a plane extending through the radiation source, and (ii) a plane extending through the radiation source and including a lateral edge extremity of the leaf closest to the beam axis. 5. The method as claimed in claim 1, comprising:storing values of the projected width for each leaf in a lookup table, andconsulting the values stored in the lookup table when determining the shape of the radiation beam at the target position. 6. The method as claimed in claim 1, wherein the target position is defined for a subject that is moveable into and out of the radiation beam. 7. The method as claimed in claim 6, further comprising:identifying a target position in the subject;determining an initial desired beam shape to irradiate the target position;positioning the subject for irradiation by the radiation beam;positioning the leaves of the mufti-leaf collimator based on the initial beam shape at the target position;irradiating the target position with the beam;determining a revised beam shape at the target position; andadjusting the position of one or more leaves of the multi-leaf collimator to change the beam shape from the initial beam shape to the revised beam shape. 8. The method as claimed in claim 7, wherein the radiation source is at a first position when the initial beam is determined and at a second position when the revised beam shape is determined, the first position being different from the second position. 9. A radiotherapy system comprising:a radiation source for projecting the radiation beam towards a target position along a beam axis;a multi-leaf collimator disposed between the radiation source and the target position, the multi-leaf collimator comprising a defocused array of moveable leaves configured to be positioned to intersect and block a part of the radiation beam so as to define the shape of the radiation beam at the target position, each leaf having a plane extending through a plane perpendicular to the beam axis and a width extending in a direction perpendicular to the plane of the respective leaf, wherein at least a first leaf and a second leaf are aligned such that the plane of the first leaf converges with the plane of the second leaf at a line, wherein a point within the line is displaced laterally from the beam axis; anda control system for controlling the multi-leaf collimator to move the leaves to provide a desired beam shape at the target position;wherein the control system is configured to:determine a projected width, with respect to the radiation beam, for each leaf positioned to intersect the radiation beam, the projected width being, greater than a width of the respective leaf, wherein the determined projected width for a leaf further from the beam axis is wider than the determined projected width for a leaf closer to the beam axis;determine the shape of the radiation beam at the target position using, the projected width of at least one leaf;position the leaves of the multi-leaf collimator based on the determined shape of the radiation beam; andirradiate the target position with the beam. 10. The system as claimed in claim 9, wherein all of the leaves of the mufti-leaf collimator have substantially the same width. 11. The system as claimed in claim 9, wherein the control system is configured to:determine a progressively wider projected width for a leaf as a distance between the leaf and the beam axis increases. 12. The system as claimed in claim 9, wherein the control system is configured to:determine the position of an edge of the radiation beam at the target position using the projected width of at least one leaf. 13. The system as claimed in claim 9, wherein the projected width is determined as a distance, in the plane perpendicular to the beam axis and passing through the leaf, between (i) a lateral edge extremity of the leaf furthest from the beam axis in a plane extending through the radiation source, and (ii) a plane extending through the radiation source and including a lateral edge extremity of the leaf closest to the beam axis. 14. The system as claimed in claim 9, wherein the control system is configured to:store values of the projected width for each leaf in a lookup table, andconsult the values stored in the lookup table when determining the shape of the radiation beam at the target position. 15. The system as claimed in claim 9, wherein the target position is defined for a subject that is moveable into and out of the radiation beam. 16. The system as claimed in claim 15, wherein the control system is configured to:identify a target position in the subject;determine an initial desired beam shape to irradiate the target position;position the subject for irradiation by the radiation beam;position the leaves of the multi-leaf collimator based on the initial beam shape at the target position;irradiate the target position with the beam;determine a revised beam shape at the target position; andadjust the position of one or more leaves of the multi-leaf collimator to change the beam shape from the initial beam shape to the revised beam shape. 17. The system as claimed in claim 16, wherein the radiation source is at a first position when the initial beam is determined and at a second position when the revised beam shape is determined, the first position being different from the second position.