The invention is related to a method and apparatus for verifying the beam range in a target irradiated with a charged hadron beam, such as a proton beam. The beam range is the location of the Bragg peak in the target, being the location where the largest portion of the dose is delivered. The method utilizes a prompt gamma camera provided with a slit-shaped opening, so as to be able to produced a 1-dimensional profile of the dose distribution along the beam line. The camera is mounted with the slit oriented perpendicularly to the beam line. The method comprises the steps of calculating a position of the camera with respect to a target, for a plurality of beam energies and spots to be irradiated. The method further comprises the steps of verifying the beam range for said plurality of spots, and delivering a value representative of the difference between the estimated beam range and the actual beam range. The apparatus of the invention is provided with a positioning module for positioning the camera.