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 each of the plurality of diseases requires a different maximal energy to achieve its associated therapeutic dose of radiation than the maximal energy required to achieve the associated therapeutic dose of radiation for another 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.