Sloping field compensation for cobalt therapy

An accurate and simple compensating method of sloping field have been devised for effective dose planning. Theexperimental formula, induced with linear absorption coefficient and measured values, shown in formula(5), isapplied accurately to sloping compensation of wide region depneds on source skin distance, field size, absorptioncoefficient and angle of incidence. According to this method, the distance of shifting for the normal isodosecurve, "h", is varied by the distance of shifting between the normal and sloping surface, "d", shown in Fig. 1. Weneed more simple compensating factor within the permitting error in clinical practice. It is possible to obtainethe reasonable copensating factor by analysing the sloping field calculated by the experimental formula. Theresults of this analysis are shown in Figs. 3-5, comparing with the shift methods of 1/2d. 2/3d and 3/4d. Inradiation therapy it may be recommended to use"Compensating factor" indicated in Table 1 to correct for thesloping field.