โ Scribed by Gerald J. Hine; Milton Friedman
No coin nor oath required. For personal study only.
ADIATION dosimetry had been stabilized R around 200to 250-kv. X-ray machines and radium sources until about 10 years ago, when the higher energy radiation sources emerged from trial status to routine clinical use. The common apparatus are 1to 2-Mv X-ray machines (Van de Graaff or resonant transformer generators), 4to 6-Mv linear accelerators, radioactive cobalt (Coeo) teletherapy apparatus, 2-to 50-Mev electron beam machines, and 24-Mv betatrons. Other new modes of radiation are employed for investigative purposes only.
Most of the physical dosimetry problems of high energy radiation beams have been fairly well explored. The effects of these high energy radiations differ in many cases from those produced by comparable doses of 250-kv. X rays.
Thus, additional factors must be considered to obtain a good correlation between the absorbed dose and the biological effect.
Clinical radiation dosimetry encompasses, in addition to the measured dose, a number of modifying variables, such as: the relative biological effectiveness (RBE) of the selected type of radiation as compared to 200-kv. X rays; the over-all time interval between the start and the completion of treatment; the size, number, and geometric shape of the treatment ports, the angle of incidence on the skin and the choice of stationary fields or rotation techniques: and, finally, the site of irradiation in the patient, the status of the disease, and the tissue response in the patient. All these variables determine the biological effect of an absorbed radiation dose. This might be expressed by the following relationship:
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