Principles of blood irradiation, dose validation, and quality control

he use of irradiated blood components to prevent graft-versus-host disease in susceptible patients has increased dramatically in the past sevT eral years. Irradiation eliminates the proliferative capacity of lymphocytes present in red cell, platelet, and freshly collected plasma component^.'-^ After penetrating blood components, the photons delivered by a radiation beam cause the formation of electrically charged particles or secondary electrons. These electrons damage the DNA of lymphocytes, either by direct interaction or by reacting initially with cell water to form free radicals. The damaged lymphocytes are unable to proliferate in the host and therefore cannot mediate transfusion-associated graft-versus-host disease. The photons used to irradiate blood components are generated by one of two methods, using either a gamma-ray beam or an x-ray beam. Gamma rays contain photons generated by the decay of radioactive isotopes such as cesium137 (Cs-137) or cobalt-60 (co-60). These isotopes can be positioned inside lead-enclosed chambers in dedicated irradiation instruments. X-rays are photon beams generated mechanically by teletherapy devices that accelerate electrons to very high speeds, directing them to a metallic target such as tungsten and generating a photon beam as a result of this collision. The linear accelerator is an example of the kind of instrument that generates x-rays as an irradiation source. There are no physical differences between gamma rays and x-rays; they exhibit the same radiation characteristics and damage lymphocytes in a similar manner.4