Dosage determination for rotation therapy in the horizontal plane.

Abstract

In one aspect of moving-field x-ray therapy, the beam may be made to go completely or partially around the body, either by rotating the patient around an axis perpendicular to the axis of the beam or by moving the x-ray tube on the circumference of a circle centered on an axis through the patient. This may be described as “vertical” or “perpendicular” or “long-axis” rotation; the method has been employed by a considerable number of radiologists, and dosage calculations are fairly straightforward (1). In another procedure, the patient (usually in a recumbent position) is rotated about an axis at an angle to the beam, or the beam describes a path over one surface of the patient. This is usually designated “horizontal” or “short-axis” rotation therapy. Each of these methods is advantageous in certain types of case. By means of long-axis rotation, small deep-seated lesions can be given high doses, while surrounding tissues and skin are spared. The method is applicable to either spherical lesions or elongated ones such as those in the esophagus. Short-axis rotation is more useful for lesions considerably closer to one surface of the body, although deeper ones may be treated by two rotation setups cross-firing from opposite sides of the body. It is usually suitable only for spherical or relatively flat lesions. The matter of dosage determination for horizontal rotation therapy has been well analyzed by Green, Jennings, and Bush (2). For the basic calculations reported in this paper, their method has been followed, with some modifications (see Appendix III). For approximate calculations, however, simpler methods have been developed, in order to permit rapid decision as to the applicability of the procedure in any special case. The problems involved in developing dosage determination methods arise from variations dose due the oblique passage the beam through the body. This is evident from consideration of an instantaneous position in this type of therapy, as shown at the left in Figure 1. The tilted beam obviously delivers a considerably higher dose of radiation in the upper part of the lesion, both because this is nearer the target than the lower part, and because the x-rays reaching this region have traversed more air and less tissue than those going to the lower portions. Jennings has pointed out that the inequality due to the air space can be eliminated by interposing a wax cone, as shown on the right in the figure; this furthermore provides a nearly flat surface perpendicular to the beam direction, so that normal static isodose charts are immediately applicable. The target-distance factor can be corrected by the use of a wedge filter, as indicated. When a circular field is used without a wedge filter, isodose surfaces are always symmetrical about the central axis of the beam; hence, all longitudinal sections of the beam show the same isodose diagram.