On the accuracy of monomer/polymer gel dosimetry in the proximity of a high-dose-rate Ir-192 source

The aim of this work was to investigate the applicability of MR-based polym er gel dosimetry to measure the absorbed dose distribution at short distanc e from an iridium-192 brachytherapy point source. In this paper, different methodological problems that may result in significant errors in the measur ed dose distribution are discussed. First of all the extent to which physic ochemical mechanisms alter the dose response is discussed. The most importa nt among these are the oxygen permeability of the catheter material and mon omer-diffusion-related effects during irradiation. The effect of oxygen on the dose-R-2 curve has been determined quantitatively and an oxygen map is performed using a well-defined external irradiation beam. The effect of dif fusion of monomer during irradiation has been simulated. Another contributi on of errors is related to magnetic susceptibility differences between the catheter and the gel during scanning the irradiated gel. The magnetic field distortion has been mapped by using both an experimental MRI technique and by simulation. Moreover, in constructing a dose-versus-distance curve by p olar averaging, the sensitivity to the exact distance between source and po int of measurement and to partial volume effects is illustrated. An optimiz ation program is proposed to determine the location of the source on a sub- pixel scale.