Teletherapy
Over 200 teletherapy units are now in use in the United States for treatment of patients by using very high intensity sources of cobalt-60 (usually) or cesium-137. Units carrying sources with intensities of more than a thousand curies are common.
The cobalt-60 unit at the M. D. Anderson Hospital and Tumor Institute in Houston, Texas, employs a 3000-curie source. This unit has a mechanism that allows for rotation therapy about a stationary patient. Many different treatment positions are possible. This patient, shown in position for therapy, has above her chest an auxiliary diaphragm that consists of an expanded metal tray on which blocks of either tungsten or lead are placed to absorb gamma rays and thus shape the field of treatment. In this case they allow for irradiation of the portions of the neck and chest delineated by the lines visible on the patient.
Since a curie is the amount of radioactivity in a gram of radium that is in equilibrium with its decay products, a 1000-curie source is comparable to 2 pounds of pure radium. Neglecting for the moment the scarcity and enormous cost of that much radium (millions of dollars), we have to consider that it would be large in volume and consequently difficult to apply. Radiation from such a quantity cannot be focussed; consequently, either much of it will fall upon healthy tissue surrounding the cancer or much of it will be wasted if a narrow passage through the shield is aimed at the tumor. In contrast, a tiny cobalt source provides just as much radiation and more if it can be brought to bear upon the exact spot to be treated.
Most interesting of all is the principle by which internal cancers can be treated with a minimum of damage to the skin. Deep x-irradiation has always been the approved treatment for deep-lying cancers, but until recently this required very cumbersome units. With the modern rotational device shown in the diagram, a very narrow beam is aimed at the patient while the source is mounted upon a carrier that revolves completely around him. The patient is positioned carefully so that the lesion to be treated is exactly at the center of the circular path of the carrier. The result is that the beam strikes its internal target during the entire circular orbit, but the same amount of radiation is spread out over a belt of skin and tissue all the way around the patient. The damage to any one skin cell is minimized. The advantage of this device over an earlier device, in which the patient was revolved in a stationary beam, is that the mechanical equipment is much simpler.