Approximation for decrement value

Several formulas to obtain the contour of the isodose curve in the treatment planning with the aid of electric computer have been reported. Orchard reported the basic idea of decrement line to acquire the isodose charts from the small amounts of experimental data without loss of accuracy. Sterling et al. have approximated the off-axis ratios of cobalt 60 γ-ray beam with cumulative normal probability function. When the treatment planning is carried out using the minicomputer, it is practically reasonable to apply the simple and accurate equation. This paper describes a simple and practical method of the approximation for the decrement value. The dose profiles of the beam along the line parpendicular to the central axis at the depth of 5, 10 and 15 cm for the therapy unit of cobalt 60 and 6 MV linear accelerator were measured. As these profiles were similar to the sigmoidal curve, Verhulst's logistic curve was used to approximate the curves of dose profiles. The differences between the measured value and the calculated value of our method were three percent at the most. As compared with the other method to calculate the decrement value, this approximation is simple and can be easily applied to the mini-computer. (author)