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Nuclear medicine is based around a fundamental concept of administering a radiopharmaceutical to a patient with the aim of achieving a diagnostic or therapeutic result. Like all medical procedures, the risk vs. benefit ratio needs to be understood, optimized and acceptable. What makes internal dosimetry more complicated than other types of radiation dosimetry calculations is the close proximity of the radioactivity source to organs and tissues, as well as the complexities involved in characterizing the source of the radioactivity and dosimetric volumes of interest, e.g. the activity distribution relative to the organs and tissues of interest.
There is no gold standard system for determining internal dosimetry, but there are several developed resources and methodologies available for estimating the dosimetric impact of clinical nuclear medicine practices.
Nuclear medicine dosimetry is used for clinical applications (both imaging and therapeutic procedures), radiation safety, and as well as for new drug development.
The mean absorbed dose to an organ from a radiopharmaceutical is dependent on the characteristics of both the radionuclide and the pharmaceutical in terms of the type and amount of radiation emitted and the spatial and temporal distribution of the radiopharmaceutical in the body.The methodology most commonly used for internal dosimetry in humans in contemporary literature is the MIRD formalism, developed by the society of Nuclear Medicine The other established methodology is that developed by the ICRP, which essentially is very similar to the approach used in the MIRD methodology. These approaches provide the methodology, terms, and equations necessary to determine radiodosimetry estimates, and can be used for situations where there is a known patient geometry.
The general equation for general absorbed dose calculations is the following:
For internal dosimetry, the numerator, E, is not simply derived because it depends on the activity and distribution of the radioactivity source relative to the volume of dosimetric interest and is thus not easily described. For the sake of allowing manageable discrete calculations, sources and targets are usually summarized as source and target organs, or volumes or interest. In general E is calculated for every source affecting a target, and should account for 100% of relevant radioactivity. The value of E should incorporate:
- Number of disintegrations from the source (usually summarized as source organs)
- Energy of the disintegrations
- Fraction of energy absorbed in target per disintegration of source (based on geometry, attenuation, and absorption properties)
Dose is often reported as calculated for particular/critical organs using absorbed dose (D) or equivalent dose (ED). Overall dose to an individual can be reported as a total body dose or effective dose. The MIRD pamphlet No. 21 provides an excellent clarification on the different values and nomenclatures which have been used in the field of radiopharmaceutical dosimetry, as well as an overview of the current state of nuclear medicine dosimetry. This article also encourages the nuclear medicine community to standardize the use of "equivalent dose" and "effective dose" for comparative evaluations of potential risks for nuclear medicine procedures.
Traditional dose calculation algorithms have calculated dosimetry for standard phantoms - human models with known geometries. More recently, with the development of Monte Carlo and dose point kernel techniques, patient specific dosimetry is beginning to be studied and employed.
Introduction to References
Because of the complexities involved in nuclear medicine dosimetry, as well as the array of methods and literature which has been presented, there are several organizations and web resources which provide overviews of the field - to a greater extent than what has been presented here. Interested readers are advised to visit the following websites: the Medical Internal Radiation Dose (MIRD) Committee (SNM), the RAdiation Dose Assessment Resource (RADAR), the Radiation protection of patients (RPoP -IAEA) - Nuclear Medicine section the International Commission on Radiological Protection (ICRP), and the National Council on Radiation Protection and Measurements (NCRP).