The present invention is related to a water phantom for measuring and determining the dose distribution of a particle beam or photon radiation beam produced by a radiation therapy apparatus. The water phantom is provided with a water tank (2) and an acquisition detector (5). Said acquisition detector (5) comprises a plurality of sensors for providing measurements of the dose distribution in a plurality of points in an area. Subsequent measurements are performed varying each time the water level within the water tank (2), until the dose distribution in the entire water tank volume is obtained.
