The assessment of radiation absorbed dose is a fundamental issue to plan nuclear medicine therapies with radionuclides. The dose distribution depends on the biokinetics of the radiopharmaceutical and from the physical decay scheme of the radionuclide carried by the molecule. While the physical properties of each nuclide are well known from experimental data, the biodistribution of the radiopharmaceutical within the patient's body depends on the dynamic biologic pathway that, in turn, is governed by the physio-pathologic role of the molecule, by the characteristics of the patient, by the type and stage of the disease, and by the route of administration.The activity distribution must be sampled several times post-administration, by means of planar or tomographic (SPECT or PET) imaging. Tomographic techniques are rapidly substituting planar whole body imaging, since, thanks also to the accurate attenuation correction and image coregistration brought by a simultaneous CT scan, they reach a spatial resolution and an accuracy in activity quantification unprecedented.After a general review on the planar and tomographic image quantification techniques, we focus on the dosimetric models, going from the different anthropomorphic models proposed for the whole human body or specific anatomic districts, to 3D techniques based on voxel dose factors, convolution of dose point-kernels and direct Monte Carlo computation. Particular emphasis will be given to the contribution of Monte Carlo simulation to the development of new and more accurate dosimetric and microdosimetric models for internal dosimetry, especially in relationship with the radiobiological models of the effect of the different particulate radiations employed.We describe the role, specific protocols and obtainable results in the main nuclear medicine therapies such as the 131-I therapy of thyroid diseases, the therapy of neuroendocrine tumors (NET) with somatostatin analogs labeled with beta- or Auger-emitters, and the therapy of non-Hodgkin lymphomas with beta-labeled monoclonal antibodies, focusing on dose-efficacy relationships and on the limiting of side effects to other potentially critical organs. © 2013 by Nova Science Publishers, Inc. All rights reserved.
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