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The value of absorbed dose represents the radiation energy deposited in a tissue per unit mass. This quantity has units of grays (1 Gy = 1 J/kg). This value is very important in all medical procedures which use radiation because it serves as an indicator to predict tissue response. The dosimetry performed in nuclear medicine is called internal dosimetry. This name illustrates the fact that the doses are deposited by particles emitted from radionuclides located within the body. Hence, in order to properly calculate the absorbed dose to a tissue during a nuclear medicine procedure, we need to accurately assess the radionuclide distribution in the body, how many times those radionuclides decay, and how much of the energy they emit gets deposited in the tissue of interest.

The tissues where the radionuclides are located are called source tissues and the tissues where the energy is deposited are called target tissues. The amount of radionuclide that localizes in a source tissue is the uptake.   Source tissues can also be target tissues. Self-dose is the dose deposited in a source tissue by the radioactive decays occurring within it.  Source and target tissues are not implicitly a whole organ, they can be any region of interest as defined by the scientist (a group of organs, organ subregions, specific cell groups, or even cell organelles or molecules such as DNA).

Currently, the uncertainty associated with internal dosimetry calculations is very large compared to that of external therapy calculations. Expected errors in external beam doses are less than 5% while the most accurate internal dosimetry estimates one can currently achieve have errors of at least 20%.    

It is also important to note, that the term “dose” is used both in reference to the amount of activity administered to the patient and the absorbed dose imparted to a tissue.  The meaning of the word in each instance can be inferred by the context. The term “Effective dose” is also used in internal dosimetry documents. Effective dose and absorbed dose are calculated using the same procedure, only radiation weighting factors accounting for the differences between doses imparted by different types of particles and tissue weighting factors accounting for the radiation sensitivity of different organs are included in the effective dose calculations. For more information refer to the documents listed in the bibliography section.