Flux Glenn, Bardies Manuel, Monsieurs Myriam, Savolainen Sauli, Strands Sven-Erik, Lassmann Michael
Department of Physics, Royal Marsden Hospital, London, UK.
Z Med Phys. 2006;16(1):47-59. doi: 10.1078/0939-3889-00291.
Targeted radionuclide therapy (TRT) is an increasingly used treatment modality for a range of cancers. To date, few treatments have involved the use of dosimetry either to plan treatment or to retrospectively ascertain the absorbed dose delivered during treatment. Also the correlation between absorbed dose and biological effect has been difficult to establish. Tomographic methods permit the determination of the activity volume on a macroscopic scale at different time points. Proper attenuation correction in tomographic imaging requires a patient-specific attenuation map. This can be obtained from scintillation-camera transmission scanning, CT or by using segmented scatter-emission images. Attenuation corrections can be performed either on the projection images, on the reconstructed images, or as part of an iterative reconstruction method. The problem of image quantification for therapy radionuclides, particularly for I-131, is exacerbated by the fact that most cameras are optimised for diagnostic imaging with Tc-99m. In addition, problems may arise when high activities are to be measured due to count losses and mis-positioned events, because of insufficient pile-up and dead time correction methods. Sufficient image quantification, however is only possible if all effects that degrade the quantitative content of the image have been corrected for. Monte Carlo simulations are an appealing tool that can help to model interactions occurring in the patient or in the detector system. This is helpful to develop and test correction techniques, or to help to define detectors better suited to quantitative imaging. PET is probably the most accurate imaging method for the determination of activity concentrations in tissue. PET imaging can be considered for pre-therapeutic treatment planning but ideally requires the use of a radioisotope from the same element as that used for treatment (e.g. I-124 for I-131; Y-86 for Y-90). Problems, however are that--some of the positron emitting isotopes have a shorter half-life--non-standard quantification procedures have to be performed--the availability of the radiopharmaceutical is presently limited; Many 3D-tools and -techniques are now available to the physicist and clinician to enable absorbed dose calculations to both target and critical organs-at-risk. The challenge now facing nuclear medicine is to enable this methodology to be routinely available to the clinic, to ensure common standard operating procedures between centres and in particular to correlate response criteria with absorbed dose estimates.
靶向放射性核素治疗（TRT）是一种越来越多地用于治疗多种癌症的方法。迄今为止，很少有治疗涉及使用剂量测定法来规划治疗或回顾性确定治疗期间所给予的吸收剂量。而且，吸收剂量与生物学效应之间的相关性一直难以确立。断层扫描方法允许在宏观尺度上于不同时间点确定活度体积。断层成像中的适当衰减校正需要患者特异性的衰减图。这可以通过闪烁相机透射扫描、CT或使用分段散射发射图像来获得。衰减校正可以在投影图像上、重建图像上进行，或者作为迭代重建方法的一部分来进行。对于治疗性放射性核素，尤其是I - 131的图像定量问题，因大多数相机是针对用Tc - 99m进行诊断成像而优化这一事实而加剧。此外，由于计数损失和定位错误事件，在测量高活度时可能会出现问题，这是因为堆积和死时间校正方法不足。然而，只有对所有降低图像定量内容的效应都进行了校正，才有可能进行充分的图像定量。蒙特卡罗模拟是一种有吸引力的工具，可有助于对患者体内或探测器系统中发生的相互作用进行建模。这有助于开发和测试校正技术，或有助于定义更适合定量成像的探测器。PET可能是用于确定组织中活度浓度的最准确成像方法。PET成像可用于治疗前的治疗规划，但理想情况下需要使用与治疗所用元素相同的放射性同位素（例如，用于I - 131的I - 124；用于Y - 90的Y - 86）。然而，问题在于——一些发射正电子的同位素半衰期较短——必须执行非标准的定量程序——目前放射性药物的可获得性有限；现在物理学家和临床医生可以使用许多三维工具和技术来计算靶器官和关键危险器官的吸收剂量。核医学目前面临的挑战是使这种方法能够在临床上常规可用，确保各中心之间有共同的标准操作程序，特别是要将反应标准与吸收剂量估计值相关联。
