A method for the measurement of dose distribution in radiotherapy apparatus comprising an apparatus for generating a beam of therapeutic radiation and an imaging apparatus, one or both apparatuses being mounted to a rotatable gantry adapted in use to rotate around a bore of the radiotherapy apparatus and around an isocentre therein, the method comprising: placing a detector holder within a tank, the detector holder comprising a radio-opaque marker and a visual reference point, the marker and the reference point being separated and in a fixed spatial relationship to each other, and the tank in use containing a liquid having a surface level uppermost; locating the tank within the bore such that the detector holder is in the vicinity of the isocentre; rotating the gantry and monitoring the position of the detector holder using the imaging apparatus to determine the position of the marker relative to the isocentre; displacing the detector holder 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 until the surface of the liquid is at the same level as the reference point. Features of the detector holder and tank are also described.