Convergent Beam Irradiator

When using sources of high-energy gamma or x-radiation for teletherapy, it is necessary to provide a means of accurately positioning the beam relative to the patient. This may be done in a variety of ways. The choice of method usually involves some compromise between cost and utility. The most common method has been to provide a supporting framework for the source of radiation which allows one or more straight line motions in Cartesian co-ordinates and one or more rotational freedoms about an axis passing through or near to the source. Equipment has lately been designed which allows movement of the source in a centered arc of a circle on the tumor (1). With the patient fixed, all required beam directions for deep therapy are provided by a source moving on the surface of a sphere, with the tumor at the center of the sphere and the beam directed radially inward to this center. Since the integral dose is considerably increased if the beam passes through the patient at a small angle to the long axis of the body, such beams should be avoided. The more useful beam directions are provided by a source moving on that portion of a sphere as illustrated graphically in Figure 1. In general, it is preferable to orient beams by moving the source rather than the patient and to adjust the equipment until the beam of radiation is correctly aligned. Once this has been done so that the center of the tumor coincides with the center of the hypothetical sphere of source movement, one may use any number of different beam directions without further change in the position of the patient. This sort of approach has led us to design the deep therapy equipment shown in Figure 2. The photograph reproduced is of the first installation at the Francis Delafield Hospital (New York) in May 1953. General Description of the Equipment The source of radiation for which this equipment was designed is a radioactive isotope delivering from 20 to 60 r per minute at 1 meter, of approximately 1.2-mev gamma radiation, which corresponds to about 3-million-volt x-rays. The cobalt-60 source used in the first equipment gives 18.7 r per minute at 1 meter. A protective housing for the source with a shutter to turn the beam on and off is mounted at one end of a curved arm. At the other end of this arm is mounted a combined counterweight and primary radiation shield, centered on the axis of the beam. This assembly is supported on a horizontal shaft whose axis is 4 feet above floor level. The attachment between the curved arm and the shaft is such that relative movement is possible between them, the curved arm being capable of rotation in a limited arc, centered on the horizontal axis of the shaft, with the arc of movement lying in the arm. This movement results in a circular planetary motion of the opposed source housing and shield-counterweight about the same point in space as the curved arm. The horizontal shaft can be rotated to give a circular motion of the sources about its axis.