The present invention is related to a water phantom for measuring and determining the dose distribution of radiation produced by a particle beam or photon radiation beam comprising: a water tank; means for varying the water level in said water tank; an acquisition detector positioned in a fixed position related to the water tank opposite to the beam, wherein said acquisition detector is a two dimensional detector comprising a plurality of sensors and capable of simultaneously measuring the dose in a plurality of points in an area. Subsequent measurements are performed varying each time the water level within the water tank, until the dose distribution in the entire volume of the water tank is obtained.