[Consideration of inhomogeneities in irradiation planning. I. Influence of inhomogeneities on course of depth dose curve in water for cobalt60 gamma rays].

In radiation therapy, the focal doses during irradiation of a tumor are based on the values for water, since water has almost the same absorption coefficient as muscular tissue, even for different kinds and energies of radiation. But calculation of the tumor dose will become inaccurate if inhomogeneities in the ray path are not considered such as fat, bones, plaster, metal plates, Küntscher nail, endoprothesis. These materials, having a density phi different from water, represent inhomogeneities relative to water with regard to absorption of high-energy radiations. The experiments yielded the following results: All measurements revealed that the change in the course of the depth dose curve caused by inhomogeneities in water depends essentially on the density phi and on the thickness d of the inhomogeneity. If the density phi of the inhomogeneity exceeds one, a shift of the depth dose curve in water results in direction to the surface: if the density phi is smaller than one, the depth dose curve will move towards greater depth because of the inhomogeneity. With Co-60 gamma radiation, the shift of the depth dose curve in water due to an inhomogeneity occurs almost parallel. A correlation obtained empirically allows a calculation of the extent of the shift, the depth dose is subjected to, for different inhomogeneities. With the density of the inhomogeneity phi2 and the thickness d, the shift V in water (phi1=1) will result from V=0.5 (phi2-1) d (cm) in direction to the surface if phi2 greater than 1, towards greater depth, if phi2 less than 1.