A field-shaping device for use in short wave length radiation therapy.

Abstract

It is the purpose of this paper to describe a device, designed by one of the authors (S. G. C.) by which small-portal radiation therapy of the type currently delivered with orthovoltage equipment to irregularly shaped portals (1) may be carried out with either supervoltage x-ray or cobalt-60 apparatus. This device has been in continuous use for the last eight years and has proved to be quite satisfactory. Because of the increased tolerance of the skin at the higher voltages, the use of relatively small, well defined, shaped portals has not been as strongly emphasized with this type of therapy as with the use of orthovoltage equipment. Until very recently, there has been no attempt to use any type of asymmetrically shaped portal for irradiation of short wave length. The more recent models of the Van de Graaff 2,000,000-volt generator are equipped with optional beam-shaping accessories but these are intended for use with relatively large-field therapy. The “Theratron F” cobalt teletherapy unit of the Commercial Products Division of Atomic Energy of Canada, Limited, has been released with a satellite diaphragm fixed to the machine rather than the treatment couch. The advantages attending a simple yet effective means of controlling and shaping the irradiated volume should be immediately clear to all who are interested in short wave length therapy. Small-portal shaped-beam therapy has been most useful in our hands for malignant disease in and about the head and neck (Fig. 1) and for breast cancer (Fig. 2). It has been our practice for many years to employ such fields in treating neoplasms of the oral cavity, accessory nasal sinuses, pharynx, and neck, and their attendant metastases, by both orthovoltage and supervoltage radiation (1, 2). As is well known, it is possible to deliver much larger doses of radiation with a greater degree of safety through small portals, with shielding of vital structures than when large fields are used. It is imperative that certain structures, such as the cervical spinal cord, the brain, and the orbital contents, be protected from irradiation, if not involved by tumor. For example, in treating a cancer of the maxillary antrum in which the orbit is not involved, we have for some years made use of shaped antro-ethmoid portals, using supervoltage irradiation prior to radical resection (3). Portal films made with the device described here have shown a well defined beam conforming to the outline of the desired portal with a very small penumbra, and the orbital structures are adequately protected (Fig. 3). The adequacy of this protection is borne out by the fact that patients have normal eyes many years following treatment delivered in this manner. The device which we use for detailed shaping of a short wave length radiation beam consists of a platform with a Lucite surface.