Abstract: The present invention provides a method for producing a broad range of therapeutic radiation energy levels with a source of radiation that is operated to produce photons or particles at a first energy that are directed at a treatment site for a predetermined absorbed dose. The linear accelerator is then operated to produce photons or particles at a second energy directed at the treatment site for another predetermined absorbed dose. The appropriate selection of the first and second absorbed doses yields an absorbed dose at the treatment site equivalent to the dose produced by photons having an energy that is intermediate of the first and second energies. Thus, a dual energy linear accelerator may be operated according to the method to yield a continuous range of energies.