Radiotherapy and imaging apparatus

A method of operating a radiotherapy apparatus, the radiotherapy apparatus comprising:

• a source of therapeutic radiation mounted on a chassis, the chassis being rotatable about a rotation axis and the source being adapted to emit a beam of radiation along a beam axis that intersects with the rotation axis;
• a patient support, moveable along a translation axis;
• a set of magnetic coils located on either side of the beam, for establishing a magnetic field at a point of intersection, spaced from the point of intersection along a first direction;
• the translation axis, the rotation axis, and the first direction being substantially parallel;
• and
• a multi-leaf collimator comprising a plurality of elongate leaves disposed with their longitudinal directions substantially aligned with the first direction and movable in that direction, the multi-leaf collimator being rotationally fixed in its orientation with respect to the source of radiation such that the elongate leaves are movable only in the first direction, and wherein the source of radiation is adapted to emit a beam of radiation having a first extent in the first direction, and a second, greater extent in a second direction normal to the first direction, the method comprising:
• irradiating a target region of a patient on the patient support with a therapeutic radiation beam;
• acquiring imaging data of the target region;
• and
• adapting the positions of the elongate leaves in dependence on the imaging data.