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Photon Therapy Photons are x-rays produced outside of the nucleus. Photons are identical to gamma rays with the exception that gamma rays are x-rays produced within a nucleus when an atom transforms from a higher to a lower energy state. Cobalt-60 is one example of an atom which gives off gamma rays and it is Cobalt - 60 which was used as the source of radiation in most radiation therapy units in years past. Cobalt - 60 units are still in use in many treatment centers but their use is limited in that they can only produce x-rays (gamma rays) of an average energy of 1.25 million electron volts(MeV). The Department of Radiation Oncology at the University of Arizona does not use Cobalt - 60 in the treatment of patients. All external beam radiation therapy is delivered by linear accelerators which produce high energy photons ranging in energy between 6 million volts (Mv) and 10 Mv. These high energy photons are capable of penetrating deeply into tissues to reach deep seated tumors while actually delivering less radiation dose to superficial tissues such as skin. The actual photon energy chosen to treat a tumor depends on the characteristics of the tumor, patient and treatment technique. Sometimes the lower photon energies are preferred and sometimes the higher energy photons can be used to improve the therapeutic ratio. A single photon beam can be used to treat relatively superficial tissues such as lymph nodes in the neck and supraclavicular regions. More often, two or more intersecting beams of radiation are directed at a tumor from different directions.
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