A method for the measurement of dose distribution in radiotherapy apparatus comprising a combined treatment and imaging apparatus, one or both apparatuses being mounted to a gantry adapted to rotate around a bore of the radiotherapy apparatus and around an isocentre therein. The method comprises placing a detector holder 22 within a tank 16, the detector holder comprising a radio-opaque marker and a visual reference point in a separate and fixed spatial relationship to each other, and the tank 16 in use containing a liquid； locating the tank 16 within the bore such that the detector holder 22 is in the vicinity of the isocentre； rotating the gantry and monitoring the position of the detector holder 22 using the imaging apparatus to determine the position of the marker relative to the isocentre； displacing the detector holder 22 vertically such that the reference point is positioned at a point level with a desired surface level of the liquid, and adding liquid to or removing liquid from the tank 16 until the surface of the liquid is at the same level as the reference point. Features of the detector holder 22 and tank 16 are also described.
