Department of Radiotherapy, NHS Grampian, Aberdeen, United Kingdom.
Biomed Phys Eng Express. 2020 Jan 30;6(1):015031. doi: 10.1088/2057-1976/ab6ecf.
Leipzig-style skin brachytherapy applicators are an excellent choice for the treatment of small surface lesions, since they can be used with a high dose rate source to produce a tightly constrained treatment field on the desired area of the skin. The dosimetry of these applicators is challenging to independently verify due to their small dimensions, complex energy spectrum and steep dose gradients. In particular the close proximity of the brachytherapy source to the treatment region is cause for concern, since small variations in the position of the radioactive source may significantly affect the resulting dose distribution. The aim of this work was to assess the dosimetry of these applicators using three independently techniques and use these results to examine the effect of variation in source position on the dose distribution. Simulation of different sized applicators in conjunction with a Gammamed + Ir source was performed using the EGSnrc Monte Carlo code. Dose distributions at the prescription depth and at the surface generated by Monte Carlo were compared to the outputs of a commercially available treatment planning system and measurements using radiochromic film. Source displacements of up to 0.5 mm in the vertical direction, 0.65 mm in the horizontal direction, and rotations of the source by up to 5° were all simulated. Changes in dose of over 6% at the prescription point and reductions in coverage at the 100% isodose level of several millimetres were observed even for small shifts of the source from its intended position. This work demonstrates that variation in the position of the radiation source is the dominant source of uncertainty in the use of these types of applicators. Centres wishing to perform treatments using these applicators are advised to take steps to control the uncertainty and ensure it remains at an acceptable level.
莱比锡式皮肤近距离治疗敷贴器是治疗小面积皮肤病变的绝佳选择，因为它们可以与高剂量率源一起使用，在皮肤的所需区域产生紧密受限的治疗场。由于这些敷贴器的尺寸小、能谱复杂且剂量梯度陡峭，因此其剂量学很难独立验证。特别是近距离治疗源与治疗区域的接近度令人担忧，因为放射性源位置的微小变化可能会显著影响最终的剂量分布。这项工作的目的是使用三种独立的技术评估这些敷贴器的剂量学，并利用这些结果研究源位置变化对剂量分布的影响。使用 EGSnrc 蒙特卡罗代码模拟了不同尺寸的敷贴器与 Gammamed+Ir 源的结合。通过蒙特卡罗生成的在处方深度处和表面处的剂量分布与商业可用的治疗计划系统的输出以及使用放射性色迹胶片进行的测量进行了比较。模拟了源在垂直方向上的最大 0.5 毫米、水平方向上的最大 0.65 毫米的位移，以及源最大 5°的旋转。即使源位置有微小的偏移，在处方点的剂量变化超过 6%，在 100%等剂量线处的覆盖范围减少了几个毫米。这项工作表明，辐射源位置的变化是使用这些类型的敷贴器的主要不确定性来源。建议希望使用这些敷贴器进行治疗的中心采取措施控制不确定性并确保其保持在可接受的水平。
