A water tank apparatus 100 for use with a radiotherapy system. Tank apparatus 100 comprising a tank structure formed by a base wall 102, side walls 104, end walls 106 and a top wall 108. In top wall 108, near one of the end-walls 106, is an aperture surrounded by an upstanding rim 112. Fixed within the tank structure is a sensor mounting body with formations that allow a radiation sensor 120 to be fixedly mounted so as to detect radiation at a point 124 equidistant from the base, top and side-walls 102,108,104. Also a method of detecting radiation from a radiotherapy system involves positioning apparatus 100 so radiation detection point 124 is at the isocentre of the radiotherapy system’s beam and measuring the radiation with the beam directed through the top, base and side-walls 108,102,104. The method may further comprise placing a polymer block on top wall 108, or adjacent a side-wall 104 and measuring radiation with the beam directed through the polymer block, the respective top or side-wall 108,104 and detection-point 124. The polymer-block is described as solid water as it’s prepared to attenuate a radiotherapy beam in substantially the same way as an equivalent volume of water.
