An assessment of bone marrow and bone endosteum dosimetry methods for photon sources
An assessment of bone marrow and bone endosteum dosimetry methods for photon sources
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DOI：
10.1088/0031-9155/51/21/001
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发表时间：
2006-11-07
影响因子：
3.5
通讯作者：
Bolch, Wesley E.
中科院分区：
文献类型：
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作者：
Lee, Choonik;Lee, Choonsik;Bolch, Wesley E.
骨骼系统相当复杂和微观的组织学结构通常限制了人们在剂量学评估中准确模拟该组织的能力。因此，必须做出各种假设来评估从外部和内部光子到红色（或造血活性）骨髓的辐射敏感组织和骨骼内膜的成骨组织的吸收剂量。这些不同的光子骨骼剂量测定方法还没有被相互比较，部分原因是缺乏一个现实的参考模型，可以提供二次电子粒子输运的高分辨率三维几何形状。在本研究中，Shah等人（2005 J. Nucl.）开发的配对图像辐射输运（PIRT）模型。利用Med. 45 344)来评估理想化的单能光子平行光束入射到这些骨骼区域的每入射光子的吸收剂量。然后将PIRT模型的结果用作局部参考，与其他方法的吸收剂量进行比较。对于红骨髓剂量测定，考虑了四种近似技术：(1)ORNL/TM-8381中提出的剂量响应函数法（DRF法），(2)质能吸收系数比法（双参数MEAC法），(3)额外使用King和Spiers （1985 Br.）提出的能量依赖剂量增强因子的MEAC法。J. Radiol. 58 345)（三参数MEAC方法），以及(4)通过使用图像特定CT数在体素级应用的三参数MEAC方法（CTN方法）。对于骨内膜（即骨表面），比较了两种近似技术：(1)骨表面的DRF方法和(2)均匀骨近似（HBA）方法。在每种情况下，都假定当地参考标准为PIRT模型的参考标准。研究中使用了四种内部结构明显不同的离体骨标本：头盖骨、腰椎、髋部和左中肋骨，均取自一具66岁男性尸体（体重指数22.7 kg m(-2)）。这些骨骼部位的高分辨率CT图像用于构建蒙特卡洛辐射传输的计算体素模型。研究结果表明，具有厚皮质区和厚小梁的骨骼部位，如头盖骨，在低光子能量下提供相当大的光束衰减，这在基于均匀骨组织结构（DRF， MEAC， HBA）的方法中没有得到适当的解释。对于骨髓剂量评估，CTN方法在较宽的光子能量范围内与PIRT模型结果最一致，而HBA方法在评估100 keV以上能量的骨内皮剂量时与PIRT模型结果更一致。在能量低于50 keV时，用DRF法近似地获得了更好的骨表面剂量。光子能量超过1-3 MeV时，在RBM剂量评估中，只有在PIRT模型中考虑了相当大的二次电子逸出，因为其他方法要么假设海绵状膜（DRF）的无限膨胀，要么假设存在电荷-粒子平衡（MEAC， CTN）。
The rather complex and microscopic histological structure of the skeletal system generally limits one's ability to accurately model this tissue during dosimetric evaluations. Consequently, various assumptions must be made to evaluate the absorbed dose from external and internal photons to the radiosensitive tissues of the red (or haematopoietically active) bone marrow and the osteogenic tissues of the skeletal endosteum. These various methods for photon skeletal dosimetry have not been inter-compared, partly due to the lack of a realistic reference model that can provide a high-resolution three-dimensional geometry for secondary electron particle transport. In the present study, the paired-image radiation transport (PIRT) model developed by Shah et al (2005 J. Nucl. Med. 45 344) was utilized to evaluate the absorbed dose per incident photon fluence to these skeletal regions from idealized parallel beams of monoenergetic photons. The PIRT model results were then used as a local reference against which absorbed doses via other methods were compared. For red bone marrow dosimetry, four approximate techniques were considered: (1) the dose response function method (DRF method) presented in ORNL/TM-8381, (2) the mass-energy absorption coefficient ratio method (two-parameter MEAC method), (3) the MEAC method with the additional use of energy-dependent dose enhancement factors from King and Spiers (1985 Br. J. Radiol. 58 345) (three-parameter MEAC method), and (4) the three-parameter MEAC method applied at the voxel level through the use image-specific CT numbers (CTN method). For the bone endosteum (i.e., bone surfaces), two approximate techniques were compared: (1) the DRF method for bone surfaces and (2) the homogeneous bone approximation (HBA) method. In each case, the local reference standard was assumed to be that of the PIRT model. Four different ex vivo bone specimens with distinctively different internal structures were used in the study: the cranium, the lumbar vertebra, the os coxae and the left middle rib, each excised from a 66 year male cadaver (body mass index, 22.7 kg m(-2)). High-resolution CT images of these skeletal sites were used to construct computational voxel models for Monte Carlo radiation transport. Study results indicated that skeletal sites with thick cortical regions and thick trabeculae such as in the cranium provide considerable beam attenuation at low photon energies, which is not properly accounted for in methods based on a homogeneous skeletal tissue structure (DRF, MEAC, HBA). For bone marrow dose assessment, the CTN method showed the best agreement with PIRT model results over a broad range of photon energies, while the HBA method showed better agreement with the PIRT model in assessing bone endosteum dose at energies above 100 keV. Bone surface doses were better approximately by the DRF method at energies below 50 keV. Considerable secondary electron escape at photon energies over 1-3 MeV were accounted for in RBM dose assessment only in the PIRT model, as the other methods presume either an infinite expanse of spongiosa (DRF) or the existence of charge-particle equilibrium (MEAC, CTN).