J Crayton Pruitt Family Department of Biomedical Engineering, University of Florida, Gainesville, FL, United States of America.
Medical Physics Program, College of Medicine, University of Florida, Gainesville, FL, United States of America.
Phys Med Biol. 2020 Dec 2;65(23):235025. doi: 10.1088/1361-6560/abb1db.
Skeletal scintigraphy is most performed in pediatric patients using the radiopharmaceutical Tc labelled methylene diphosphonate (Tc-MDP). Reference biokinetic models for Tc-MDP indicate 50% of the administered activity is uniformly localized to the interior bone surfaces (trabecular and cortical regions), yet imaging data clearly show some preferential uptake to the epiphyseal growth plates of the long bones. To explore the dosimetric consequences of these regional activity concentrations, we have modified mesh-type computational phantoms of the International Commission on Radiological Protection (ICRP) reference pediatric series to explicitly include geometric models of the epiphyseal growth plates (2 mm in thickness) within the left/right, distal/proximal ends of the humeri, radii, ulnae, femora, tibia, and fibulae. Bone mineral activity from the ICRP Publication 128 biokinetic model for Tc-MDP (ICRP 2015) was then partitioned to the growth plates at values of 0.5%, 4.4%, 8.3%, 12.2%, 16.1%, and 20%. Radiation transport simulations were performed to compute Tc S-values and organ dose coefficients to the soft tissues and to bone site-specific regions of spongiosa. As the percentage of bone activity assigned to the growth plates was increased (from 0.5% to 20%), absorbed doses to the soft tissue organs, active bone marrow, bone endosteum (BE), as well as the detriment-weighted dose, were shown to decrease from their nominal values (no substantial growth plate activity), while epiphyseal plate self-doses increased. In the 15 year old male phantom, moving from 0.5% to 20% relative bone activity within the epiphyseal plates resulted in a 15% reduction in active marrow (AM) and BE dose, a 10% reduction in mean soft tissue and detriment-weighted dose, and a 6.3-fold increase in epiphyseal plate self-dose. In the newborn female phantom, we observed a 18% decrease in AM and BE dose, a 10% decrease in mean soft tissue dose, a 15% decrease in detriment-weighted dose, and 12.8-fold increase in epiphyseal plate self-dose. Increases (to 3 mm) and decreases (to 1 mm) in the assumed growth plate thickness of our models were shown to impact only the growth plate self-dose. Future work in differential quantification of Tc-MDP activity-growth plates versus other bone surfaces-is required to provide clinically realistic data on activity partitioning as a function of patient age, and perhaps skeletal site. The phantom series presented here may be used to develop more optimized age-related guidance on Tc-MDP administered activities to children.
骨骼闪烁显像术在儿科患者中最常使用放射性核素标记的亚甲基二膦酸盐（Tc-MDP）进行。Tc-MDP 的参考生物动力学模型表明，50%的给药活性均匀定位于内部骨表面（小梁和皮质区域），但成像数据清楚地显示出一些对长骨骨骺生长板的优先摄取。为了探索这些区域活性浓度的剂量学后果，我们修改了国际辐射防护委员会（ICRP）参考儿科系列的网格型计算体模，以明确包括骺板生长板（2 毫米厚）的几何模型，位于左右肱骨、桡骨、尺骨、股骨、胫骨和腓骨的远/近端。然后，将 Tc-MDP 的 ICRP 出版物 128 生物动力学模型（ICRP 2015）中的骨矿物质活性分配给生长板，分配值为 0.5%、4.4%、8.3%、12.2%、16.1%和 20%。进行辐射传输模拟以计算 Tc S 值和软组织器官剂量系数，以及海绵骨的骨部位特异性区域。随着分配给生长板的骨活性百分比增加（从 0.5%增加到 20%），软组织器官、活性骨髓、骨内膜（BE）的吸收剂量以及受损加权剂量均显示出低于其名义值（无明显生长板活性），而骺板自剂量增加。在 15 岁男性体模中，将骺板内相对骨活性从 0.5%增加到 20%，导致活性骨髓（AM）和 BE 剂量减少 15%，平均软组织和受损加权剂量减少 10%，骺板自剂量增加 6.3 倍。在新生女性体模中，我们观察到 AM 和 BE 剂量减少 18%，平均软组织剂量减少 10%，受损加权剂量减少 15%，骺板自剂量增加 12.8 倍。我们模型中假设的生长板厚度的增加（增加到 3 毫米）和减少（减少到 1 毫米）仅影响生长板自剂量。需要对 Tc-MDP 活性-生长板与其他骨表面的差异定量进行进一步研究，以提供与患者年龄和骨骼部位相关的活性分配的临床实际数据。这里呈现的体模系列可用于制定更优化的与年龄相关的 Tc-MDP 管理活动指南，供儿童使用。
