Abstract: The present invention therefore provides a method of treating the surface of an item for a radiotherapy apparatus, comprising the steps of machining into a planar reflective surface of the item a stepped profile, each step having a first surface angled to the planar reflective surface such that, in use, the first surface reflects light incident on the planar reflective surface away from the isocenter of the radiotherapy apparatus and a second surface angled with respect to the planar reflective surface and the first surface such that, in use, the second surface is shadowed from light incident on the planar reflective surface by the first surface, the steps having a depth which does not exceed 2.15% of the total depth of the item.

Abstract: A collimator set for a radiotherapy apparatus comprises, in sequence, an aperture collimator, a multi-leaf collimator with a pair of opposing arrays of elongate leaves each moveable longitudinally in a Y direction, and a leaf edge collimator, the aperture collimator being adapted to collimate the beam in the X and Y direction to a first extent, and the leaf edge collimator being adapted to further collimate the extent of the beam in the Y direction to a second and therefore lesser extend. This means that to close a pair of opposing leaves, they are moved to their minimum separation, with the gap being convered by the leaf edge collimator. The MLC is after the aperture collimator, so in combination with thin leaves, the MLC leaves will project a much reduced leaf width at the isocentre of the radiotherapy apparatus and collimation of the radiation field by fractional leaf widths becomes unnecessary. A radiotherapy apparatus comprising a collimator set as defined above is also disclosed.