MONTE-CARLO CALCULATED DOSE DISTRIBUTION FOR ENDOVASCULAR HDR BRACHYTHERAPY WITH IR-192

Background and purpose: For endovascular HDR brachytherapy, very thin sources are required and the dose is specified at a short distance to the source centre down to 1.5 mm. The source which is used in the Nucl etron HDR Selectron stepping source afterloader is treated by most dos e calculation algorithms in clinical use as a point source, although i ts dimensions are large compared to these dose specification distances . Furthermore, inaccuracies might be introduced because consecutive dw ell positions show an overlap of sources if the step size is smaller t han the active length of the source. Materials and methods: In order t o quantify these inaccuracies, we used the EGS4 Monte Carlo code to ge nerate the dose distribution with 0.5 mm spatial resolution for a sing le source. From this, translation and superposition were used to calcu late dose distributions for multiple dwell positions. The results are compared with those of other Monte Carlo computations and of a commerc ial brachytherapy planning system. Results and conclusions: Our Monte Carlo calculations showed that secondary electrons have no relevant in fluence on the dose distribution and that errors up to 25% are made wh en using the point source approximation for irradiations with a single dwell position. However, when three or more dwell positions are used with equal dwell times, the total error becomes negligibly small becau se the errors from subsequent dwell positions compensate each other. A t distances larger than 5 mm, there is a good match between the Monte Carlo data and those of point source algorithms for all clinical relev ant cases. (C) 1997 Elsevier Science Ireland Ltd.