Cancer may be described as the runaway growth of cells. The secretions of the pituitary gland serve to stimulate cell reproduction, so it was reasoned that destruction of this gland might well slow down growth of a tumor elsewhere in the body. The trouble was that the pituitary is small and located at the base of the brain. Surgical removal had brought dramatic relief (not cure) to many patients, but the surgery itself was difficult and hazardous. Tiny yttrium-90 oxide beads, glasslike in nature, can be implanted directly in the gland with much less difficulty and risk, and do the work of destroying the gland with little damage to its surroundings. The key to the success of yttrium-90 is the fact that it is a beta-emitter, and beta rays have so little penetrating power that their effect is limited to the immediate area of the implant. 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. [Illustration: _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. [Illustration: Diagram of teletherapy unit]