Nuclear Medicine Center, Université Catholique de Louvain, Brussels, Belgium.
J Nucl Med. 2010 Dec;51(12):1969-73. doi: 10.2967/jnumed.110.080093. Epub 2010 Nov 15.
Accurate dosimetry in (90)Y peptide receptor radionuclide therapy (PRRT) helps to optimize the injected activity, to prevent kidney or red marrow toxicity, while giving the highest absorbed dose to tumors. The aim of this study was to evaluate whether direct (90)Y bismuth germanate or lutetium yttrium orthosilicate time-of-flight PET was accurate enough to provide dosimetry estimates suitable to (90)Y PRRT.
To overcome the statistical uncertainty arising from the low (90)Y positron counting rate, the computation of the cortex mean-absorbed dose was divided into 4 steps: delineation of the cortex volume of interest (VOI) on the CT scan, determination of the recovery coefficient from the cortex VOI using the point-spread function of the whole imaging process, determination of the mean cortex-absorbed dose per unit cumulated activity in the cortex (S(cortex←cortex) value) from the cortex VOI using a (90)Y voxel S value kernel, and determination of the number of decays in the cortex VOI from the PET reconstruction. Our 4-step method was evaluated using an anthropomorphic abdominal phantom containing a fillable kidney phantom based on the MIRD kidney model. Vertebrae with an attenuation similar to that of bone were also modeled. Two tumors were modeled by 7-mL hollow acrylic spheres and the spleen by a plastic bag. Activities corresponded to typical tissue uptake in a first (90)Y-DOTATOC cycle of 4.4 GBq, considered as free of significant renal toxicity. Eight successive 45-min scans were acquired on both systems.
Both PET systems were successful in determining absorbed dose to modeled tumors but failed to provide accurate red marrow dosimetry. Renal cortex dosimetry was reproducible for both PET systems, with an accuracy of 3% for the bismuth germanate system but only 18% for the lutetium yttrium orthosilicate time-of-flight system, which was hindered by the natural radioactivity of the crystal, especially in the most attenuated area of the kidney.
This study supports the use of direct (90)Y PET of the first PRRT cycle to assess the kidney-absorbed dose and optimize the injected activity of the following cycles.
评估直接 (90)Y 铋锗酸盐或硅酸镥钇时间飞行 PET 是否足够准确，以提供适合 (90)Y 肽受体放射性核素治疗 (PRRT) 的剂量估计。
为了克服由于低 (90)Y 正电子计数率引起的统计不确定性，皮质平均吸收剂量的计算分为 4 个步骤：在 CT 扫描上描绘皮质感兴趣区 (VOI)，使用整个成像过程的点扩散函数从皮质 VOI 确定恢复系数，使用 (90)Y 体素 S 值核从皮质 VOI 确定皮质每单位累积活性的平均皮质吸收剂量 (S(cortex←cortex) 值)，以及从 PET 重建中确定皮质 VOI 中的衰变数。我们的 4 步方法使用基于 MIRD 肾脏模型的可填充肾脏模型的人体腹部模拟体进行了评估。还模拟了具有类似于骨骼衰减的椎骨。通过 7-mL 空心丙烯酸球和塑料袋分别模拟两个肿瘤和脾脏。活动对应于典型的组织摄取，即第一个 (90)Y-DOTATOC 周期的 4.4GBq，被认为没有明显的肾毒性。在两个系统上连续进行了 8 次 45 分钟的扫描。
两种 PET 系统都成功地确定了模型肿瘤的吸收剂量，但未能提供准确的红骨髓剂量。对于两种 PET 系统，皮质肾剂量均具有可重复性，铋锗酸盐系统的准确性为 3%，而硅酸镥钇时间飞行系统的准确性仅为 18%，这主要是由于晶体的天然放射性，尤其是在肾脏衰减最大的区域。
这项研究支持在第一个 PRRT 周期中使用直接 (90)Y PET 来评估肾脏吸收剂量，并优化后续周期的注射活性。
