基于蒙特卡罗EGS4算法的核素内照射吸收剂量计算方法探讨

Primary study on calculation method of absorption dose of radionuclide therapy by Monte Carlo EGS4 method

目的以蒙特卡罗 EGS4算法(Monte Carlo EGS4,MC EGS4)为基础,用时序性 SPECT/CT 检查探讨核素内照射治疗吸收剂量的计算方法。方法用体模标定^(153)Sm 放射性浓度与 SPECT图像灰度值的关系;用 RMI467型 CT 体模标定不同组织物理密度与 CT 图像灰度值的关系;优化 MCEGS4 计算程序。以此为基础,通过时序性 SPECT/CT 检查和累积尿液的放射性测定,计算4例肿瘤多发骨转移患者^(153)Sm-乙二胺四亚甲基膦酸(EDTMP,按体重注射24.1 MBq/kg)内照射治疗后不同靶器官的三维吸收剂量分布和病灶、骨髓、脊髓、盆腔性腺组织的吸收剂量。结果 SPECT 和 CT 图像的灰度值分别与^(153)Sm 放射性浓度和组织物理密度之间存在线性对应关系(P<0.05)。多发骨转移癌患者骨转移灶的^(153)Sm-EDTMP 吸收剂量分布明显不均,放射性累积中心点吸收剂量最高,边缘区域剂量降低。1例患者最高点内照射吸收剂量率为4.3×10^(-8)Gy·s^(-1),左髂骨转移灶最高吸收剂量约为5.6 Gy,病灶边缘吸收剂量为2.0 Gy。其他3例患者病灶最高点吸收剂量率分别为4.5×10^(-8),3.5×10^(8),3.8×10^(-8)Gy·s^(-1)。结论基于 MC EGS4算法,用时序性 SPECT/CT 可计算核素内照射治疗患者的病灶和其他靶器官吸收剂量及其三维分布。

Objective Medical Internal Radiation Dose （MIRD） proposed by International Committee Radiation Protection （ICRP） has provided a method for overall radiation dose calculation. The limitation of MIRD was that it was unable to evaluate the individual absorption dose. The aim of this study was to try to explore novel method, based on Monte Carlo, for internal irradiation absorption dose calculation which was evolved from time-ordered SPECT/CT infusion imaging. Methods ASPECT body phantom was used to define the relationship between the radioactive concentration of 153^Sm-ethylene diamine tetramethylene phosphonate （EDTMP） and the gray values of SPECT images. RMI467 CT body phantom was applied to establish the relationship between physical densities and CT values of X-CT images. For patient studies, each case was sequentially scanned using SPECT/CT. Meanwhile, their urine samples were detected for the radioactivity of 153^Sm-EDTMP （initial injection dose was 24. 1 MBq/kg） as to fit the calculation formula of optimized Monte Carlo EGS4 method. Results The linear relationships were found either between gray values of SPECT imaging and 153^Sm radioactivity or between the CT values of X-CT scans and physical densities （P 〈 0. 05）. Individual dose calculation indicated that the absorption doses by patients with multiple bone metastases were heterogeneously distributed, with peak doses targeting at the highest uptake points and lower doses at margin areas. The maximum absorption dose ratio and dose of left hipbone metastasis focus of one patient were 4.3×10^-8Gy·s^-1 and 5.6 Gy, the dose of focus margin was 2.0 Gy; and other 3 patients＇ absorption dose ratios were 4.5 ×10^-8, 3.3 ×10^-8, 3.8 ×10^-8Gy·s^-1 respectively. Conclusion The internal absorption dose could be accurately calculated by Monte Carlo method based on time-ordered SPECT/CT images.