Absorbed Dose Estimation and Measurement
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Radiopharmaceuticals (RPs) have a unique attribute of emitting ionizing radiation that enables both diagnosis and therapy in nuclear medicine. Local ionization is in addition to any other physical, chemical, or immunological effects of the RP. The emitted particles deposit energy in various organs and may cause various forms of destruction. These results can occur relatively soon after irradiation (direct damage) or sometime later (delayed effects). Historically, the lowest-order estimate of the likelihood of such damage has been the concept of absorbed radiation dose. Rather than repeat this long phrase, we will sometimes use the term dose as an alternative in the following discussions. Dose is essentially an energy density in the target tissue. It is important, therefore, that methods for estimation and even measurement be considered for RP absorbed dose in vivo. Likewise, we must realize that direct dose measurement, because of its invasive nature, is generally not applicable in either animals or patients. Mathematical methods of estimating absorbed dose are seen as the fundamental practical technique for use in determining dangers associated with ionizing radiation. Several such methods are described in this chapter. In addition, we will show that some measurements are possible using humanoid phantoms or with skin detectors on patients.