Purpose: This study aims to provide a comparison between MCNPX and MIRDcell calculations for self-dose and cross-dose for three therapeutic isotopes used in internal radiotherapy (Lu-177, I-131 and Y-90) and to develop a multi-cellular geometric model to simulate an in vitro scenario.

Materials and Methods: The self- and cross-dose to individual cell nuclei were assessed by Monte Carlo N-Particle eXtended (MCNPX). A close-packed cubic cell arrangement was assumed with the same amount of radioactivity per cell. Various cell sizes and subcellular distributions of radioactivity (nucleus, cytoplasm and cell membrane) were simulated. S values were obtained by MIRDcell for comparison. A Python 3.4 program was used to generate random cell coordinates in order to build a complex model that takes certain real conditions (cell size and cluster size) into account.

Results: The relative differences of MCNPX versus MIRD S values (S_self) ranged from 2.88 to 10.10% for Lu-177; from 0 to 8.41% for I-131 and from 2.80 to 9.58% for Y-90. The relative differences of MCNPX versus MIRDcell cross-dose S values were 3.6%−15.7% for a sphere. The ratio of S_cross max to S_self decreased for Lu-177 and I-131 with increasing cell size. The source localization within the cells had no significant impact on the cross-dosing. For single cells, the subcellular location of the source had an effect on S_self.

Conclusions: MCNPX and MIRD cell-calculated S values showed good agreement. The model provided could be used to predict the biological effect caused by emitted radiation from therapeutic radionuclides at the cellular level.

目的：本研究旨在比较内部放射治疗中使用的三种治疗性同位素（Lu-177，I-131和Y-90）的自剂量和交叉剂量的MCNPX和MIRDcell计算之间的比较，并开发出多细胞几何模型来模拟体外情况。

材料和方法：通过蒙特卡罗N粒子扩展（MCNPX）评估单个细胞核的自剂量和交叉剂量。假设一个密排的立方单元排列，每个单元具有相同的放射性。模拟了各种细胞大小和放射性的亚细胞分布（核，细胞质和细胞膜）。通过MIRDcell获得S值进行比较。为了建立一个复杂模型，该模型考虑了某些实际条件（像元大小和簇大小），使用Python 3.4程序生成随机像元坐标。

结果： Lu-177的MCNPX与MIRD S值的相对差异（S _self）为2.88％至10.10％；I-131为0至8.41％，Y-90为2.80至9.58％。对于一个球体，MCNPX与MIRDcell交叉剂量S值的相对差异为3.6％-15.7％。随着细胞尺寸的增加，Lu-177和I-131的S _cross max与S _self的比率降低。细胞内的源定位对交叉给药没有显着影响。对于单细胞，来源的亚细胞位置对S _self有影响。

结论： MCNPX和MIRD细胞计算的S值显示出良好的一致性。提供的模型可用于预测在细胞水平上由治疗性放射性核素发出的辐射引起的生物学效应。