Monte Carlo calculated dose distribution for endovascular HDR brachytherapy with Ir-192

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 Nucletron HDR Selectron stepping source afterloader is treated by most dose calculation algorithms in clinical use as a point source, although its dimensions are large compared to these dose specification distances. Furthermore, inaccuracies might be introduced because consecutive dwell positions show an overlap of sources if the step size is smaller than the active length of the source.

In order to quantify these inaccuracies, we used the EGS4 Monte Carlo code to generate the dose distribution with 0.5 mm spatial resolution for a single source. From this, translation and superposition were used to calculate dose distributions for multiple dwell positions. The results are compared with those of other Monte Carlo computations and of a commercial brachytherapy planning system.

Our Monte Carlo calculations showed that secondary electrons have no relevant influence on the dose distribution and that errors up to 25% are made when 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 because the errors from subsequent dwell positions compensate each other. At distances larger than 5 mm, there is a good match between the Monte Carlo data and those of point source algorithms for all clinical relevant cases.