A method for determining a beam parameter of an unflattened photon beam generated by an accelerator includes measuring radiation dose values in the plane perpendicular to the beam propagation direction, determining the extension of the unflattened beam using a definition of the beam extension of a flattened beam. The method also includes normalizing the radiation dose values, such that essentially the same value for the extension of the unflattened beam is obtained as would be obtained if the beam was flattened and determining the beam parameter of the unflattened beam using a beam parameter definition of a flattened beam.
