Shah Amish P, Bolch Wesley E, Rajon Didier A, Patton Phillip W, Jokisch Derek W
Department of Biomedical Engineering, University of Florida, Gainesville, Florida 3261-8300, USA.
J Nucl Med. 2005 Feb;46(2):344-53.
Toxicity of the hematopoietically active bone marrow continues to be a primary limitation in radionuclide therapies of cancer. Improved techniques for patient-specific skeletal dosimetry are thus crucial to the development of dose-response relationships needed to optimize these therapies (i.e., avoid both marrow toxicity and tumor underdosing). Current clinical methods of skeletal dose assessment rely heavily on a single set of bone and marrow cavity chord-length distributions in which particle energy deposition is tracked within an infinite extent of trabecular spongiosa, with no allowance for particle escape to cortical bone. In the present study, we introduce a paired-image radiation transport (PIRT) model that can provide a more realistic 3-dimensional geometry for particle transport of the skeletal site at both microscopic and macroscopic levels of its histology.
Ex vivo CT scans were acquired of the lumbar vertebra and right proximal femur excised from a 66-y male cadaver (body mass index, 22.7 kg m(-2)). For both skeletal sites, regions of trabecular spongiosa and cortical bone were identified and segmented. Physical sections of interior spongiosa were then taken and subjected to nuclear magnetic resonance (NMR) microscopy. Voxels within the resulting NMR microimages were segmented and labeled into regions of bone trabeculae, endosteum, active marrow, and inactive marrow. The PIRT methodology was then implemented within the EGSnrc radiation transport code, whereby electrons of various initial energies are simultaneously tracked within both the ex vivo CT macroimage and the NMR microimage of the skeletal site.
At electron initial energies greater than 50-200 keV, a divergence in absorbed fractions to active marrow is noted between PIRT model simulations and those estimated under infinite spongiosa transport techniques. Calculations of radionuclide S values under both methodologies imply that current chord-based models used in clinical skeletal dosimetry can overestimate dose to active bone marrow in these 2 skeletal sites by approximately 4%-23% for low-energy beta-emitters ((33)P, (169)Er, and (177)Lu), by approximately 4%-25% for intermediate-energy beta-emitters ((153)Sm, (186)Re, and (89)Sr), and by approximately 11%-30% for high-energy beta-emitters ((32)P, (188)Re, and (90)Y).
The PIRT methodology allows for detailed modeling of the 3D macrostructure of individual marrow-containing bones within the skeleton, thus permitting improved estimates of absorbed fractions and radionuclide S values for intermediate-to-high beta-emitters.
造血活跃骨髓的毒性仍然是癌症放射性核素治疗的主要限制因素。因此,改进针对患者的骨骼剂量测定技术对于建立优化这些治疗所需的剂量反应关系(即避免骨髓毒性和肿瘤剂量不足)至关重要。目前的骨骼剂量评估临床方法严重依赖于一组单一的骨和骨髓腔弦长分布,其中在无限范围的小梁骨松质内跟踪粒子能量沉积,不考虑粒子逃逸到皮质骨。在本研究中,我们引入了一种配对图像辐射传输(PIRT)模型,该模型可以在骨骼部位组织学的微观和宏观层面为粒子传输提供更逼真的三维几何结构。
对一名66岁男性尸体(体重指数,22.7 kg m(-2))切除的腰椎和右股骨近端进行离体CT扫描。对于这两个骨骼部位,识别并分割小梁骨松质和皮质骨区域。然后取内部骨松质的物理切片并进行核磁共振(NMR)显微镜检查。将所得NMR显微图像内的体素分割并标记为骨小梁、骨内膜、活跃骨髓和非活跃骨髓区域。然后在EGSnrc辐射传输代码中实施PIRT方法,由此在骨骼部位的离体CT宏观图像和NMR显微图像中同时跟踪各种初始能量的电子。
在电子初始能量大于50 - 200 keV时,PIRT模型模拟与无限骨松质传输技术下估计的对活跃骨髓的吸收分数存在差异。两种方法下放射性核素S值的计算表明,当前临床骨骼剂量测定中使用的基于弦的模型对于低能β发射体((33)P、(169)Er和(177)Lu)在这两个骨骼部位可能高估对活跃骨髓的剂量约4% - 23%,对于中能β发射体((153)Sm、(186)Re和(89)Sr)高估约4% - 25%,对于高能β发射体((32)P、(188)Re和(90)Y)高估约11% - 30%。
PIRT方法允许对骨骼内单个含骨髓骨骼的三维宏观结构进行详细建模,从而能够更好地估计中到高β发射体的吸收分数和放射性核素S值。