New Accessories for Precise Teletherapy with Cobalt-60 Units

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Abstract
Device Reducing the Effective Source Size Because of the large size of cobalt-60 sources, field localization radiographs taken with Co60 units are inferior in quality to those produced by accelerators. For small fields, the undesirable effect of penumbra, which is proportional to the diameter of the source, is also well known. The simple device shown in Figure 1 reduces the effective source size, improves the quality of radiographs, and trims the penumbra. It consists of a solid block which is temporarily inserted in front of the source so that the axis CC of its conical hole H coincides with the central ray of the beam. Made of heavy metal (i.e., gold, uranium, tungsten, or even lead) about 2 inches thick, the device absorbs practically all the radiation except that passing though the single hole. The effective size of the newly-created source thus equals that of the hole. The angle (φ) of the conical hole is large enough so that the insertion of the block does not affect the field. Only the penumbra, in therapy, and the so-called circle of confusion, in radiography, are reduced at the expense of the intensity of the beam. With this arrangement, a double exposure radiograph is taken or therapy is given to special fields. The time for each radiographic exposure is usually between five and fifteen seconds, depending on the activity of the source, the geometry, and the type of film used. In a Co60 unit with the source mounted on a rotating wheel, the block was inserted in the space between the end of the collimator and the source, through the collimator's aperture which was opened to the maximum field position. The collimator was then closed to form the treatment field and to prevent the block from falling out, if the unit were in the vertical beam position. The modification of design shown in Figure 2 makes this accessory an integral part of the cobalt unit. The block B, ordinarily kept in a side compartment, can be moved in and out of the beam through the rod R and the knob N. When the rod R is pulled out and turned by 90°, the pin P, resting against the head surface, prevents the spring A from pushing the block into the beam. Synchronous Protection The protection of sensitive organs in rotational therapy by synchronous rotation of absorbers has already been reported (1, 2). In the following paragraphs we shall describe the actual equipment which can be used for protection of the spinal cord and the eyes during cobalt therapy and the resulting distribution of dosage in one case. The protection of the spinal cord by a cord-shaped absorber is illustrated in Figure 3.

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