Internal Radiation Dosimetry: Models and Applications

Internal radiation dosimetry has a fundamental and growing role in planning nuclear medicine therapies with radionuclides. The principle of nuclear medicine therapy is to destroy pathologic tissues through the irradiation with the ionizing radiation emitted by properly chosen radionuclides, while preserving other organs and tissues from unnecessary exposure to the same radiation. In order to realize this result, proper pharmaceuticals are chosen with a biodistribution targeted on target tissues, and labelled with a suitably chosen radionuclide. The choice of the best radionuclide is carried on with the aim of maximizing radiation energy deposition in the target tissue during the desired treatment time. Beta-emitters are the best choice in most cases, because beta radiation has a mean range in tissue from few millimetres to few centimetres. Also used are alphaand Auger-emitters, for millimetre and sub-millimetre ranges. The absorbed dose to the target tissues as well as to other organs and tissues depends from the biokinetics of the radiopharmaceutical and from the physical decay scheme of the radionuclide employed. 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 turns is governed by the 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 distribution of radioactivity within the human body must be sampled several times post-administration, by means of planar or tomographic (SPECT or PET) imaging techniques. Tomographic techniques are rapidly substituting planar whole body imaging, since, thanks also to the accurate attenuation correction and image co-registration brought by a simultaneous CT scan, they reach a spatial resolution and an accuracy in activity quantification unprecedented. After a general introduction on dosimetric quantities and their relationships, we focus on the dosimetric anthropomorphic models. We introduce also 3D techniques based on voxel dose factors, convolution of dose point-kernels and direct Monte Carlo computation, focusing on the contribution of Monte Carlo simulation to the development of new and more accurate dosimetric and microdosimetric models for internal dosimetry.