Parsai E Ishmael, Zhang Zhengdong, Feldmeier John J
Department of Radiation Oncology, University of Toledo Health Science Campus, Toledo, OH, USA.
Brachytherapy. 2009 Jul-Sep;8(3):318-23. doi: 10.1016/j.brachy.2008.11.012. Epub 2009 Feb 12.
The commercially available brachytherapy treatment-planning systems today, usually neglects the attenuation effect from stainless steel (SS) tube when Fletcher-Suit-Delclos (FSD) is used in treatment of cervical and endometrial cancers. This could lead to potential inaccuracies in computing dwell times and dose distribution. A more accurate analysis quantifying the level of attenuation for high-dose-rate (HDR) iridium 192 radionuclide ((192)Ir) source is presented through Monte Carlo simulation verified by measurement.
In this investigation a general Monte Carlo N-Particles (MCNP) transport code was used to construct a typical geometry of FSD through simulation and compare the doses delivered to point A in Manchester System with and without the SS tubing. A quantitative assessment of inaccuracies in delivered dose vs. the computed dose is presented. In addition, this investigation expanded to examine the attenuation-corrected radial and anisotropy dose functions in a form parallel to the updated AAPM Task Group No. 43 Report (AAPM TG-43) formalism. This will delineate quantitatively the inaccuracies in dose distributions in three-dimensional space. The changes in dose deposition and distribution caused by increased attenuation coefficient resulted from presence of SS are quantified using MCNP Monte Carlo simulations in coupled photon/electron transport. The source geometry was that of the Vari Source wire model VS2000. The FSD was that of the Varian medical system. In this model, the bending angles of tandem and colpostats are 15 degrees and 120 degrees , respectively. We assigned 10 dwell positions to the tandem and 4 dwell positions to right and left colpostats or ovoids to represent a typical treatment case. Typical dose delivered to point A was determined according to Manchester dosimetry system.
Based on our computations, the reduction of dose to point A was shown to be at least 3%. So this effect presented by SS-FSD systems on patient dose is of concern.
当今市售的近距离放射治疗治疗计划系统在使用弗莱彻-休伊特-德尔克洛（FSD）治疗宫颈癌和子宫内膜癌时，通常忽略了不锈钢（SS）管的衰减效应。这可能导致计算驻留时间和剂量分布时出现潜在的不准确。通过经测量验证的蒙特卡罗模拟，给出了一种更准确的分析方法，用于量化高剂量率（HDR）铱192放射性核素（¹⁹²Ir）源的衰减水平。
在本研究中，使用通用的蒙特卡罗N粒子（MCNP）输运代码通过模拟构建FSD的典型几何结构，并比较有和没有SS管时在曼彻斯特系统中输送到A点的剂量。给出了输送剂量与计算剂量之间不准确程度的定量评估。此外，本研究扩展到以与更新后的美国医学物理学家协会第43任务组报告（AAPM TG-43）形式主义平行的形式检查衰减校正后的径向和各向异性剂量函数。这将定量描绘三维空间中剂量分布的不准确程度。使用MCNP蒙特卡罗模拟在耦合光子/电子输运中量化由SS的存在导致的衰减系数增加所引起的剂量沉积和分布变化。源几何结构为可变源线模型VS2000。FSD为瓦里安医疗系统的FSD。在该模型中，施源器和阴道柱形施源器的弯曲角度分别为15度和120度。我们为施源器分配10个驻留位置，为左右阴道柱形施源器或卵形容器分配4个驻留位置，以代表一个典型的治疗病例。根据曼彻斯特剂量测定系统确定输送到A点的典型剂量。
基于我们的计算，显示输送到A点的剂量减少至少3%。因此，SS-FSD系统对患者剂量产生的这种效应值得关注。
