The three-dimensional dose distribution of 125I seeds in tissue

The radiation treatment of brain tumours with 125I seeds requires a precise knowledge of the entire three-dimensional dose distribution of the seed. Assuming a point source distribution modified by an anisotropic factor, the absorbed energy is calculated and compared with measurements obtained using 7LiF thermoluminescence dosemeters. The measurements are carried out in air and in Mix-D. A mean relative difference of 5% is found between calculation and measurement of the dose perpendicular to the seed axis. It is concluded that the entire three-dimensional distribution of absorbed dose in tissue can be determined by a simple formula, which enables the calculation of absorbed dose at any point of interest with an uncertainty of 6%.