A METHOD FOR CALCULATING ABSORBED DOSE IN TISSUE BY Cs-137 NEEDLE USING EGS4

A method using dose rate constant and anisotropy function for calculating absorbed dose distribution in brachytherapy has been recommended by AAPM Report no.51. In 2003, the radiation treatment planning equipment with the method for obtaining the absorbed dose distribution in brachytherapy using Cs-137 needles was introduced into Department of Radiology, Nagoya University Hospital. The method beforehand requires radial dose functions g(r) and anisotropy functions F(r, θ). These functions are decided from known absorbed dose. However, it is difficult technologically to measure the absorbed dose distribution in the vicinity of the Cs-137 needle for brachytherapy. Therefore, we obtained these functions after the absorbed dose distribution had been calculated using a conventional method. The absorbed dose distribution calculated by the conventional method must be compared with the measurement one. In order to verify the accuracy of this method, we compared the absorbed dose distribution obtained with this method and that obtained with EGS4 Monte Carlo simulation. Furthermore, we examined the influence of the geometry of the needle source to the absorbed dose distribution. As a result, both absorbed dose distributions obtained by two methods were good agreement with each other. In addition, there was little influence to the dose distribution on the basis of a difference of the geometry on both ends of the needle.