It has been proposed (Young, 1967) that, for megavoltage radiation, corrections of conventional depth dose estimates for inhomogeneities in the radiation field can be calculated from the equation where TAR(A, d) is the tissue/air ratio for the given radiation for area A and depth d, d1 and d2 are the distances from the point at which the dose is being estimated to the near and far sides, respecitvely, of the inhomogeneity, and pe is the electron density of the inhomogeneity (electrons/cm3) relative to that of unit density soft tissue. This paper describes a comprehensive experimental test of the equation. Aluminium, carbon, Perspex, wood, foamed plastic and air were used as inhomogeneities. Experiments were carried out with the inhomogeneity covering the entire field and, also, with the inhomogeneity intercepting a part only of the beam. Most of the measurements were made with 60Co γ rays, but one set of measurements was made with 280 kVp X radiation to examine the possibility of using the method at lower energies to correct for air spaces and lung tissue. The measurements with 60Co radiation showed that under any circumstances likely to be encountered clinically, the calculated correction factors could be used with an error of less than 3 per cent. For 280 kVp radiation (for air and lung spaces only), the correction factors predicted by the equation were in somewhat less satisfactory agreement with experiment, but still clinically useful.
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Young et al. (1970) studied this question.