1. A method, for treating diseased tissue with radiation, comprising: selecting a target disease from among a plurality of diseases, each of the plurality of diseases having an associated therapeutic dose of radiation; determining, based on the target disease, a maximal energy in an x-ray radiation beam to be emitted from a radiotherapy system, the x-ray radiation beam delivering substantially the therapeutic dose of radiation associated with the target disease to target tissue in a patient afflicted by the target disease; wherein diseases of the plurality of diseases require a different maximal energy of the x-ray radiation beam to achieve the associated therapeutic dose of radiation than the maximal energy required to achieve the associated therapeutic dose of radiation for another disease of the plurality of diseases; and wherein the maximal energy in the x-ray radiation beam is based on the selected target disease and beam energy attenuation due to any tissue overlying the target tissue; and outputting to an output module an indication of the determined maximal energy.