Dries W J
Department of Radiotherapy, Catharina Hospital, Eindhoven, The Netherlands.
Radiother Oncol. 1997 Oct;45(1):77-82. doi: 10.1016/s0167-8140(97)00136-9.
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.
对于血管内高剂量率近距离治疗，需要非常细的源，并且剂量是在距源中心短至1.5毫米的距离处指定的。尽管与这些剂量指定距离相比，Nucletron HDR Selectron步进源后装治疗机中使用的源尺寸较大，但在临床使用的大多数剂量计算算法中，该源都被视为点源。此外，如果步长小于源的有效长度，连续的驻留位置会显示源的重叠，从而可能引入误差。
为了量化这些误差，我们使用EGS4蒙特卡罗代码以0.5毫米的空间分辨率生成单个源的剂量分布。由此，通过平移和叠加来计算多个驻留位置的剂量分布。将结果与其他蒙特卡罗计算结果以及商业近距离治疗计划系统的结果进行比较。
我们的蒙特卡罗计算表明，二次电子对剂量分布没有相关影响，并且在使用点源近似进行单驻留位置照射时会产生高达25%的误差。然而，当使用三个或更多驻留位置且驻留时间相等时，总误差变得可以忽略不计，因为后续驻留位置的误差相互补偿。在大于5毫米的距离处，对于所有临床相关情况，蒙特卡罗数据与点源算法的数据都有很好的匹配。
