Department of Radiation Physics, ERESA, Hospital General Universitario, 46014 Valencia, Spain.
Radiotherapy Department, La Fe University and Polytechnic Hospital, Valencia 46026, Spain.
Med Phys. 2014 Feb;41(2):021703. doi: 10.1118/1.4860175.
In skin high-dose-rate (HDR) brachytherapy, sources are located outside, in contact with, or implanted at some depth below the skin surface. Most treatment planning systems use the TG-43 formalism, which is based on single-source dose superposition within an infinite water medium without accounting for the true geometry in which conditions for scattered radiation are altered by the presence of air. The purpose of this study is to evaluate the dosimetric limitations of the TG-43 formalism in HDR skin brachytherapy and the potential clinical impact.
Dose rate distributions of typical configurations used in skin brachytherapy were obtained: a 5 cm × 5 cm superficial mould; a source inside a catheter located at the skin surface with and without backscatter bolus; and a typical interstitial implant consisting of an HDR source in a catheter located at a depth of 0.5 cm. Commercially available HDR(60)Co and (192)Ir sources and a hypothetical (169)Yb source were considered. The Geant4 Monte Carlo radiation transport code was used to estimate dose rate distributions for the configurations considered. These results were then compared to those obtained with the TG-43 dose calculation formalism. In particular, the influence of adding bolus material over the implant was studied.
For a 5 cm × 5 cm(192)Ir superficial mould and 0.5 cm prescription depth, dose differences in comparison to the TG-43 method were about -3%. When the source was positioned at the skin surface, dose differences were smaller than -1% for (60)Co and (192)Ir, yet -3% for (169)Yb. For the interstitial implant, dose differences at the skin surface were -7% for (60)Co, -0.6% for (192)Ir, and -2.5% for (169)Yb.
This study indicates the following: (i) for the superficial mould, no bolus is needed; (ii) when the source is in contact with the skin surface, no bolus is needed for either (60)Co and (192)Ir. For lower energy radionuclides like (169)Yb, bolus may be needed; and (iii) for the interstitial case, at least a 0.1 cm bolus is advised for (60)Co to avoid underdosing superficial target layers. For (192)Ir and (169)Yb, no bolus is needed. For those cases where no bolus is needed, its use might be detrimental as the lack of radiation scatter may be beneficial to the patient, although the 2% tolerance for dose calculation accuracy recommended in the AAPM TG-56 report is not fulfilled.
在皮肤高剂量率（HDR）近距离放射治疗中，源位于皮肤表面之外、与之接触或植入皮肤表面以下的某个深度。大多数治疗计划系统使用 TG-43 形式，该形式基于无限水介质中单源剂量叠加，而不考虑真实几何形状，其中散射辐射的条件因空气的存在而改变。本研究旨在评估 HDR 皮肤近距离放射治疗中 TG-43 形式的剂量学限制及其潜在的临床影响。
获得了皮肤近距离放射治疗中典型配置的剂量率分布：5 cm×5 cm 表面模具；位于皮肤表面的导管内的源，带有和不带后散射塞；以及由位于 0.5 cm 深度的导管中的 HDR 源组成的典型间质植入物。考虑了商用 HDR（60）Co 和（192）Ir 源以及假设的（169）Yb 源。使用 Geant4 蒙特卡罗辐射传输代码来估算所考虑配置的剂量率分布。然后将这些结果与 TG-43 剂量计算形式的结果进行比较。特别是，研究了在植入物上添加塞子材料的影响。
对于 5 cm×5 cm（192）Ir 表面模具和 0.5 cm 处方深度，与 TG-43 方法相比，剂量差异约为-3%。当源位于皮肤表面时，（60）Co 和（192）Ir 的剂量差异小于-1%，而（169）Yb 的剂量差异小于-3%。对于间质植入物，皮肤表面的剂量差异为（60）Co 为-7%，（192）Ir 为-0.6%，（169）Yb 为-2.5%。
本研究表明：（i）对于表面模具，不需要塞子；（ii）当源与皮肤表面接触时，（60）Co 和（192）Ir 不需要塞子。对于能量较低的放射性核素，如（169）Yb，可能需要塞子；（iii）对于间质情况，建议至少使用 0.1 cm 塞子来避免对浅层靶层的剂量不足。对于（60）Co，对于（192）Ir 和（169）Yb 不需要塞子。对于不需要塞子的情况，由于缺乏辐射散射可能对患者有益，因此使用塞子可能有害，尽管 AAPM TG-56 报告中推荐的剂量计算准确性 2%的容差未得到满足。
