Divoli Antigoni, Chiavassa Sophie, Ferrer Ludovic, Barbet Jacques, Flux Glenn D, Bardiès Manuel
Joint Department of Physics, Institute of Cancer Research and the Royal Marsden NHS Foundation Trust, Sutton, United Kingdom.
J Nucl Med. 2009 Feb;50(2):316-23. doi: 10.2967/jnumed.108.056705. Epub 2009 Jan 21.
Dosimetric calculations are performed with an increasing frequency before or after treatment in targeted radionuclide therapy, as well as for radiation protection purposes in diagnostic nuclear medicine. According to the MIRD committee formalism, the mean absorbed dose to a target is given by the product of the cumulated activity and a dose-conversion factor, known as the S factor. Standard S factors have been published for mathematic phantoms and for unit-density spheres. The accuracy of the results from the use of these S factors is questionable, because patient morphology can vary significantly. The aim of this work was to investigate differences between patient-specific dosimetric results obtained using Monte Carlo methodology and results obtained using S factors calculated on standard models.
The CT images of 9 patients, who ranged in size, were used. Patient-specific S factors for 131I were calculated with the MCNPX2.5.0 Monte Carlo code using a tool for personalized internal dose assessment, OEDIPE; standard S factors from OLINDA/EXM were compared against the patient-specific S factors. Furthermore, realistic biodistributions and cumulated activities for normal organs and tumors were used, and mean organ- and tumor-absorbed doses calculated with OEDIPE and OLINDA/EXM were compared.
The ratio of the standard and the patient-specific S factors were between 0.49 and 1.84 for a target distant from the source for 4 organs and 2 tumors studied as source and targets. For the case of self-irradiation, the equivalent ratio ranged between 0.45 and 2.47 and between 1.00 and 1.06 when mass correction was applied. Differences in mean absorbed doses were as high as 140% when realistic cumulated activity values were used. These values decreased to less than 26% in all cases studied when mass correction was applied to the self-irradiation given by OLINDA/EXM.
Standard S factors can yield mean absorbed doses for normal organs or tumors with a reasonable accuracy (26% for the cases studied) as compared with absorbed doses calculated with Monte Carlo, provided that they have been corrected for mass.
在靶向放射性核素治疗的治疗前或治疗后，以及在诊断核医学中出于辐射防护目的，剂量计算的执行频率越来越高。根据MIRD委员会的形式体系，靶区的平均吸收剂量由累积活度与一个剂量转换因子（称为S因子）的乘积给出。已针对数学模型和单位密度球体公布了标准S因子。使用这些S因子所得结果的准确性值得怀疑，因为患者形态可能有显著差异。这项工作的目的是研究使用蒙特卡罗方法获得的患者特异性剂量学结果与使用基于标准模型计算的S因子所得结果之间的差异。
使用了9名体型各异患者的CT图像。使用MCNPX2.5.0蒙特卡罗代码以及用于个性化内照射剂量评估的工具OEDIPE计算了131I的患者特异性S因子；将OLINDA/EXM中的标准S因子与患者特异性S因子进行了比较。此外，使用了正常器官和肿瘤的实际生物分布及累积活度，并比较了用OEDIPE和OLINDA/EXM计算的平均器官和肿瘤吸收剂量。
在所研究的作为源和靶区的4个器官和2个肿瘤中，对于远离源的靶区，标准S因子与患者特异性S因子的比值在0.49至1.84之间。对于自照射情况，当应用质量校正时，等效比值在0.45至2.47之间以及在1.00至1.06之间。当使用实际累积活度值时，平均吸收剂量的差异高达140%。当对OLINDA/EXM给出的自照射应用质量校正时，在所有研究案例中这些值降至低于26%。
与用蒙特卡罗方法计算的吸收剂量相比，标准S因子在经过质量校正后，可以以合理的准确度（在所研究案例中为26%）得出正常器官或肿瘤的平均吸收剂量。
