A Compensator for Variations in Tissue Thickness for High Energy Beams

Abstract

When high energy radiation beams are used, it is an important fact that the skin may be spared to some extent, since the dose builds up to a maximum at a depth below the surface. When we started to use a cobalt 60 beam unit, it was obvious that the technique used by us in 250 kV X-ray therapy, of building a wax platform to convert the irregular contours of a patient into a flat surface, is undesirable at higher energies, because it sacrifices the skin-sparing advantage of the build-up. Physical measurements showed that the absence of an absorber resulted in deviations from standard isodose curves of the order of 10 to 30 per cent. The following method was devised by one of us (FE) to overcome this difficulty. If, instead of being in contact with the skin, the wax block is displaced away from the skin to a distance sufficient to ensure that secondary electrons from the wax do not reach the skin, the skin-sparing effect is preserved. At the same time, the absorption along any given ray is independent of whether the wax is in contact with the skin or displaced from it, provided that the lateral dimensions of the wax block at right angles to the beam direction are reduced to allow for its change of distance from the source, while its thickness remains unaltered (Fig. 1).