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]
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.