Department of Medical Radiation Physics, Clinical Sciences, Lund, Lund University, Lund, Sweden.
Acta Oncol. 2011 Aug;50(6):981-9. doi: 10.3109/0284186X.2011.584559.
Dosimetry in radionuclide therapy estimates delivered absorbed doses to tumours and ensures that absorbed dose levels to normal organs are below tolerance levels. One procedure is to determine time-activity curves in volumes-of-interests from which the absorbed dose is estimated using SPECT with appropriate corrections for attenuation, scatter and collimator response. From corrected SPECT images the absorbed energy can be calculated by (a) assuming kinetic energy deposited in the same voxel where particles were emitted, (b) convolve with point-dose kernels or (c) use full Monte Carlo (MC) methods. A question arises which dosimetry method is optimal given the limitations in reconstruction- and quantification procedures.
Dosimetry methods (a) and (c) were evaluated by comparing dose-rate volume histograms (DrVHs) from simulated SPECT of (111)In, (177)Lu, (131)I and bremsstrahlung from (90)Y to match true dose rate images. The study used a voxel-based phantom with different tumours in the liver. SPECT reconstruction was made using an iterative OSEM method and MC dosimetry was performed using a charged-particle EGS4 program that also was used to determined true absorbed dose rate distributions for the same phantom geometry but without camera limitations.
The DrVHs obtained from SPECT differed from true DrVH mainly due to limited spatial resolution. MC dosimetry had a marginal effect because the SPECT spatial resolution is in the same order as the energy distribution caused by the electron track ranges. For (131)I, full MC dosimetry made a difference due to the additional contribution from high-energy photons. SPECT-based DrVHs differ significantly from true DrVHs unless the tumours are considerable larger than the spatial resolution.
It is important to understand limitations in quantitative SPECT images and the reasons for apparent heterogeneities since these have an impact on dose-volume histograms. A MC-based dosimetry calculation from SPECT images is not always warranted.
放射性核素治疗中的剂量学旨在估算肿瘤的吸收剂量，并确保正常器官的吸收剂量低于耐受水平。一种方法是确定感兴趣容积中的时间-活度曲线，然后使用适当的衰减、散射和准直器响应校正，通过 SPECT 估算吸收剂量。从校正后的 SPECT 图像中，可以通过（a）假设粒子发射所在的相同体素中沉积的动能，（b）与点剂量核函数卷积，或（c）使用全蒙特卡罗（MC）方法来计算吸收能量。考虑到重建和量化过程的限制，出现了一个问题，即哪种剂量学方法是最优的。
通过比较模拟（111）In、（177）Lu、（131）I 和（90）Y 韧致辐射的 SPECT 的剂量率体积直方图（DrVH），评估了剂量学方法（a）和（c）。该研究使用了一个具有不同肝脏肿瘤的体素化体模。SPECT 重建使用迭代 OSEM 方法进行，MC 剂量学使用带电粒子 EGS4 程序进行，该程序还用于确定相同体模几何形状但没有相机限制的真实吸收剂量率分布。
SPECT 获得的 DrVH 与真实 DrVH 主要由于空间分辨率有限而存在差异。MC 剂量学的影响较小，因为 SPECT 的空间分辨率与电子轨迹范围引起的能量分布相同。对于（131）I，由于高能光子的额外贡献，全 MC 剂量学产生了差异。除非肿瘤比空间分辨率大得多，否则基于 SPECT 的 DrVH 与真实 DrVH 存在显著差异。
了解定量 SPECT 图像的局限性以及出现明显异质性的原因非常重要，因为这些因素会对剂量-体积直方图产生影响。并非总是需要从 SPECT 图像进行基于 MC 的剂量计算。
