Saeedzadeh E, Sarkar S, Abbaspour Tehrani-Fard A, Ay M R, Khosravi H R, Loudos G
Department of Radiomedical Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran.
Radiat Prot Dosimetry. 2012 Jul;150(3):298-305. doi: 10.1093/rpd/ncr411. Epub 2011 Nov 7.
Various methods, such as those developed by the Medical Internal Radiation Dosimetry (MIRD) Committee of the Society of Nuclear Medicine or employing dose point kernels, have been applied to the radiation dosimetry of (131)I radionuclide therapy. However, studies have not shown a strong relationship between tumour absorbed dose and its overall therapeutic response, probably due in part to inaccuracies in activity and dose estimation. In the current study, the GATE Monte Carlo computer code was used to facilitate voxel-level radiation dosimetry for organ activities measured in an (131)I-treated thyroid cancer patient. This approach allows incorporation of the size, shape and composition of organs (in the current study, in the Zubal anthropomorphic phantom) and intra-organ and intra-tumour inhomogeneities in the activity distributions. The total activities of the tumours and their heterogeneous distributions were measured from the SPECT images to calculate the dose maps. For investigating the effect of activity distribution on dose distribution, a hypothetical homogeneous distribution of the same total activity was considered in the tumours. It was observed that the tumour mean absorbed dose rates per unit cumulated activity were 0.65E-5 and 0.61E-5 mGY MBq(-1) s(-1) for the uniform and non-uniform distributions in the tumour, respectively, which do not differ considerably. However, the dose-volume histograms (DVH) show that the tumour non-uniform activity distribution decreases the absorbed dose to portions of the tumour volume. In such a case, it can be misleading to quote the mean or maximum absorbed dose, because overall response is likely limited by the tumour volume that receives low (i.e. non-cytocidal) doses. Three-dimensional radiation dosimetry, and calculation of tumour DVHs, may lead to the derivation of clinically reliable dose-response relationships and therefore may ultimately improve treatment planning as well as response assessment for radionuclide therapy.
多种方法，如由核医学协会医学内照射剂量测定（MIRD）委员会开发的方法或采用剂量点核的方法，已应用于¹³¹I放射性核素治疗的辐射剂量测定。然而，研究并未表明肿瘤吸收剂量与其总体治疗反应之间存在密切关系，这可能部分归因于活度和剂量估计的不准确。在本研究中，使用GATE蒙特卡罗计算机代码来促进对一名¹³¹I治疗的甲状腺癌患者所测量的器官活度进行体素级辐射剂量测定。这种方法能够纳入器官的大小、形状和组成（在本研究中，采用祖巴尔人体模型）以及活度分布中的器官内和肿瘤内的不均匀性。从SPECT图像测量肿瘤的总活度及其非均匀分布，以计算剂量图。为了研究活度分布对剂量分布 的影响，在肿瘤中考虑了相同总活度的假设均匀分布。观察到，对于肿瘤中的均匀和非均匀分布，每单位累积活度的肿瘤平均吸收剂量率分别为0.65×10⁻⁵和0.61×10⁻⁵ mGY MBq⁻¹ s⁻¹，二者差异不大。然而，剂量体积直方图（DVH）表明，肿瘤活度的非均匀分布会降低肿瘤体积部分的吸收剂量。在这种情况下，引用平均或最大吸收剂量可能会产生误导，因为总体反应可能受接受低（即非杀细胞）剂量的肿瘤体积限制。三维辐射剂量测定以及肿瘤DVH的计算，可能会推导出临床可靠的剂量反应关系，因此最终可能会改善放射性核素治疗的治疗计划以及反应评估。
