Campos Luciana Tourinho, de Almeida Carlos Eduardo Veloso
Laboratório de Ciências Radiológicas (LCR/DBB/ UERJ), Rua São Francisco Xavier, 524 Maracanã, CEP: 205550, Rio de Janeiro, Brasil.
PLoS One. 2015 Sep 29;10(9):e0139032. doi: 10.1371/journal.pone.0139032. eCollection 2015.
The use of high-dose-rate brachytherapy is currently a widespread practice worldwide. The most common isotope source is 192Ir, but 60Co is also becoming available for HDR. One of main advantages of 60Co compared to 192Ir is the economic and practical benefit because of its longer half-live, which is 5.27 years. Recently, Eckert & Ziegler BEBIG, Germany, introduced a new afterloading brachytherapy machine (MultiSource®); it has the option to use either the 60Co or 192Ir HDR source. The source for the Monte Carlo calculations is the new 60Co source (model Co0.A86), which is referred to as the new BEBIG 60Co HDR source and is a modified version of the 60Co source (model GK60M21), which is also from BEBIG.
The purpose of this work is to obtain the dosimetry parameters in accordance with the AAPM TG-43U1 formalism with Monte Carlo calculations regarding the BEBIG 60Co high-dose-rate brachytherapy to investigate the required treatment-planning parameters. The geometric design and material details of the source was provided by the manufacturer and was used to define the Monte Carlo geometry. To validate the source geometry, a few dosimetry parameters had to be calculated according to the AAPM TG-43U1 formalism. The dosimetry studies included the calculation of the air kerma strength Sk, collision kerma in water along the transverse axis with an unbounded phantom, dose rate constant and radial dose function. The Monte Carlo code system that was used was EGSnrc with a new cavity code, which is a part of EGS++ that allows calculating the radial dose function around the source. The spectrum to simulate 60Co was composed of two photon energies, 1.17 and 1.33 MeV. Only the gamma part of the spectrum was used; the contribution of the electrons to the dose is negligible because of the full absorption by the stainless-steel wall around the metallic 60Co. The XCOM photon cross-section library was used in subsequent simulations, and the photoelectric effect, pair production, Rayleigh scattering and bound Compton scattering were included in the simulation. Variance reduction techniques were used to speed up the calculation and to considerably reduce the computer time. The cut-off energy was 10 keV for electrons and photons. To obtain the dose rate distributions of the source in an unbounded liquid water phantom, the source was immersed at the center of a cube phantom of 100 cm3. The liquid water density was 0.998 g/cm3, and photon histories of up to 1010 were used to obtain the results with a standard deviation of less than 0.5% (k = 1). The obtained dose rate constant for the BEBIG 60Co source was 1.108±0.001 cGyh-1U-1, which is consistent with the values in the literature. The radial dose functions were compared with the values of the consensus data set in the literature, and they are consistent with the published data for this energy range.
高剂量率近距离放射治疗目前在全球广泛应用。最常用的同位素源是192Ir，但60Co也开始用于高剂量率治疗。与192Ir相比，60Co的主要优势之一在于其半衰期更长，为5.27年，具有经济和实用效益。最近，德国的Eckert & Ziegler BEBIG公司推出了一款新的后装近距离放射治疗机（MultiSource®）；它可以选择使用60Co或192Ir高剂量率源。用于蒙特卡罗计算的源是新的60Co源（型号Co0.A86），即新的BEBIG 60Co高剂量率源，它是BEBIG公司的60Co源（型号GK60M21）的改进版本。
本研究旨在通过蒙特卡罗计算，依据美国医学物理师协会（AAPM）TG - 43U1形式主义获取BEBIG 60Co高剂量率近距离放射治疗的剂量学参数，以研究所需的治疗计划参数。源的几何设计和材料细节由制造商提供，用于定义蒙特卡罗几何结构。为验证源几何结构，必须根据AAPM TG - 43U1形式主义计算一些剂量学参数。剂量学研究包括空气比释动能强度Sk、在无界体模中沿横轴在水中的碰撞比释动能、剂量率常数和径向剂量函数的计算。所使用的蒙特卡罗代码系统是带有新腔室代码的EGSnrc，它是EGS++的一部分，可用于计算源周围的径向剂量函数。模拟60Co的能谱由1.17 MeV和1.33 MeV两种光子能量组成。仅使用能谱的伽马部分；由于金属60Co周围不锈钢壁的完全吸收，电子对剂量的贡献可忽略不计。后续模拟使用了XCOM光子截面库，模拟中包括光电效应、电子对产生、瑞利散射和束缚康普顿散射。采用方差缩减技术来加速计算并大幅减少计算机时间。电子和光子的截止能量均为10 keV。为获取源在无界液态水体模中的剂量率分布，将源浸没在一个100 cm³的立方体模中心。液态水密度为0.998 g/cm³，使用多达10¹⁰个光子历史记录来获得标准差小于0.5%（k = 1）的结果。所获得的BEBIG 60Co源的剂量率常数为1.108±0.001 cGy h⁻¹U⁻¹，与文献中的值一致。将径向剂量函数与文献中共识数据集的值进行比较，它们与该能量范围内已发表的数据一致。
