1. A computerized device for generating an exposure plan for irradiating a patient with therapeutic penetrating radiation that interacts with target tissue, which is surrounded by other tissue, to beneficially modify said target tissue, said therapeutic radiation being attenuated by said other tissue, said device comprising: a computer having access to at least one reference data set for a reference body that is independent of the patient but corresponds to a region of the patient containing said target tissue and said other tissue, said computer being configured to determine, from said at least one reference data set, reference body attenuation of said penetrating radiation in said reference body and to determine, from said reference body attenuation, a representation of region attenuation of said penetrating radiation in said region of the patient; said computer also having access to magnetic resonance data representing a patient magnetic resonance image of said region of the patient, and said computer comprising an adaptation unit that adapts said patient magnetic resonance image dependent on said representation of region attenuation determined from said reference data set; said computer being configured to determine patient-specific attenuation of said penetrating radiation upon passage through the region of the patient from the adaptation of the patient magnetic resonance image with the representation of region attenuation determined from said reference data set, and said computer being configured to generate an exposure plan dependent on said patient magnetic resonance image and said patient-specific attenuation of said penetrating radiation; and an output unit in communication with said computer at which said computer causes said exposure plan to be visually presented.