Field Shaping and Selective Protection in Megavolt Radiation Therapy

Abstract

The availability of intense sources of megavolt radiation makes possible the treatment of regional as well as localized disease with more adequate doses. These new capabilities are often best exploited by the use of large treatment fields, carefully shaped to include known tumor and local extensions and to exclude uninvolved regions. This procedure results in a more uniform dose distribution and reduces the problems of overlap inherent in the multiple field approach. Field shaping, or field outline control, is accomplished by interposing sufficient thickness of absorbing material into the beam cross section. In addition, the beam may be intensity-modulated by filters to adjust for variations in the anatomical thickness. In our megavolt x-ray equipment a light-beam localizer is used to facilitate the placing of such absorbers and filters. Double-exposure radiographs are taken to confirm and record the field location and the position of these absorbers. Fields of typical shape used in both rotational and opposing portal therapy that have evolved from a nine-year clinical investigation program will be presented. In rotational therapy it is often desirable to reduce or to minimize the dose to certain radiation-sensitive regions within the field. Several methods have been developed to accomplish such selective shielding consistent with the treatment of adjacent involved regions. Technics will be described for shielding the spinal cord and the eye during rotational therapy of surrounding and adjacent tissue. The method for providing spinal cord protection makes use of a shaped absorber rod rotating in synchronism with the patient. This rod is positioned between the target and patient and bent so that its projected size and shape cover the spinal cord in both lateral and anteroposterior projections. As the patient and absorber rod are synchronously rotated, the radiation to the cord is continuously attenuated. The degree of such selective protection depends, among other things, on the thickness and density of the absorber. By the use of high density materials such as gold, the dose of 2 MEV radiation delivered by the direct beam to the axis of the cord is reduced to half by rods of 3/8-inch diameter. To obtain equivalent attenuation with lead rods, greater thicknesses must be used. The gradient in dose around the protected region and several other factors must be taken into account in planning the treatment. Protection of the eyes can be accomplished during rotational therapy treatment by a similar method. Two horizontal parallel absorber rods directed to cover the eyes are used. These rods are of sufficient length to reduce to about 1 per cent the direct beam transmitted through them and are kept parallel as they are rotated about an appropriate axis synchronously with the patient. Throughout the rotation each absorber rod thus remains in continuous alignment with its target eye.