Larsson Erik, Strand Sven-Erik, Ljungberg Michael, Jönsson Bo-Anders
Medical Radiation Physics, Department of Clinical Sciences, Lund, Lund University, Lund, Sweden.
Cancer Biother Radiopharm. 2007 Jun;22(3):438-42. doi: 10.1089/cbr.2006.320.
Biokinetic and dosimetry studies in small animals often precede clinical radionuclide therapies. As in human studies, a reliable evaluation of therapeutic efficacy is essential and must be based on accurate dosimetry, which must be based on a realistic dosimetry model. The aim of this study was to evaluate the differences in the results when using a more anatomic realistic mouse phantom, as compared to previously mathematically described phantoms, based mainly on ellipsoids and cylinders. The difference in results from the two Monte Carlo codes, EGS4 and MCNPX 2.6a, was also evaluated.
An anatomical correct mouse phantom (Moby) was developed by Segars et al. for the evaluation and optimization of the in vivo imaging of mice. The Moby phantom is based on surfaces, which allows for an easy and flexible definition of organ sizes. It includes respiratory movements and a beating heart. It also allows for a redefinition of the location of several internal organs. The execution of the Moby program generates a three-dimensional voxel-based phantom of a specified size, which was modified and used as input for Monte Carlo simulations of absorbed fractions and S-factors. The radiation transport was simulated both with the EGS4 system and the MCNPX 2.6a code. Calculations were done for the radionuclides (18)F, (124)I, (131)I, (111)In, (177)Lu, and (90)Y. S-factors were calculated using in-house-developed IDL programs and compared with results from previously published models.
The comparison of S-factors obtained by the Moby model and mathematical phantoms showed that these, in many cases, were within the same range, whereas for some organs, they were underestimated in the mathematical phantoms. The results were closer to the more anatomically realistic phantom than to the mathematical phantoms, with some exceptions. When investing differences between MCNPX 2.6a and EGS4 using the Moby phantom, results indicated some differences in absorbed fractions for electrons. This reason may be owing to differences in the codes regarding the theory for which electron transport are simulated.
It is possible to calculate S-factors that are specific for small animals, such as mice. The Moby phantom is useful as a dosimetry model because it is anatomically realistic, but still very flexible, with 35 accurately segmented regions.
小动物的生物动力学和剂量学研究通常先于临床放射性核素治疗。与人体研究一样，对治疗效果进行可靠评估至关重要，且必须基于准确的剂量学，而这又必须基于现实的剂量学模型。本研究的目的是评估使用更具解剖学真实感的小鼠体模与先前主要基于椭球体和圆柱体的数学描述体模相比，结果的差异。还评估了两种蒙特卡罗代码EGS4和MCNPX 2.6a结果的差异。
Segars等人开发了一种解剖学正确的小鼠体模（Moby），用于评估和优化小鼠的体内成像。Moby体模基于表面，便于灵活定义器官大小。它包括呼吸运动和跳动的心脏。它还允许重新定义几个内部器官的位置。执行Moby程序会生成指定大小的基于体素的三维体模，对其进行修改后用作吸收分数和S因子蒙特卡罗模拟的输入。使用EGS4系统和MCNPX 2.6a代码模拟辐射传输。对放射性核素（18）F、（124）I、（131）I、（111）In、（177）Lu和（90）Y进行计算。使用内部开发的IDL程序计算S因子，并与先前发表模型的结果进行比较。
Moby模型和数学体模获得的S因子比较表明，在许多情况下，它们处于同一范围内，而对于某些器官，数学体模中的S因子被低估。除了一些例外情况，结果更接近解剖学上更真实的体模，而不是数学体模。使用Moby体模研究MCNPX 2.6a和EGS4之间的差异时，结果表明电子吸收分数存在一些差异。原因可能是代码在模拟电子传输的理论方面存在差异。
可以计算特定于小鼠等小动物的S因子。Moby体模作为剂量学模型很有用，因为它在解剖学上是真实的，但仍然非常灵活，有35个精确分割的区域。
