On the shape of the Task Group 43 anisotropy factor for linear brachytherapy sources at short distances

Abstract

To investigate the peak shown by the American Association of Physicists in Medicine (AAPM) Task Group 43 1D anisotropy function at short distances from the source. The 1D anisotropy function of an ideal nonencapsulated photon linear source is calculated. A simple analytical model developed to evaluate the dose because of photon point-like sources has been applied. Previously, the model has been tested by comparing the values obtained for the various Task Group 43 dosimetric functions with those calculated with the Monte Carlo code PENELOPE for three different photon energies. The model is able to reproduce the behavior of the 1D anisotropy function, describing the maximum that appears at a distance between 1 and 2mm from the source. The reason for this behavior has been identified in terms of the contributions of the source activity inside and outside the scoring sphere. Although it is not usually shown in reference data, this behavior should be taken into account for accurate dosimetric calculations.