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Dosimetry of Internal Emitters

Department of Radiology, Johns Hopkins University School of Medicine, Baltimore, Maryland

View larger version (12K): [in a new window]   FIGURE 1. Sample time-activity curve for the liver (LI) is depicted. Open circles represent measured time points. Solid line is monoexponential fit to data given by expression A(t), where A0 is injected activity, eff is effective clearance rate given by (Ln(2)/Tp + Ln(2)/Tb), and fLI is fraction of administered activity in liver, back-extrapolated to injection time. Integral of expression from t = 0 to infinity gives expression shown in box. Residence time () may be derived as shown in second box.

View larger version (33K): [in a new window]   FIGURE 2. Different tissue absorption properties of photons versus electron or -particle emissions of radionuclide are illustrated. Photons originating in liver, for example, depending on energy, can irradiate distant organs. Correspondingly, not all of photon energy emitted within source organ will be absorbed by source organ, as reflected in possible range of absorbed fraction values for photons. In contrast, great majority of electron (Auger and ß-particles) or -particle energy will be absorbed very close to emission source and within source tissue. Correspondingly, energy absorption to other tissues is negligible.

View larger version (25K): [in a new window]   FIGURE 3. MIRD schema is illustrated. Terms in absorbed dose equation that are independent of radionuclide biodistribution have been compiled and tabulated as S values. S values are tabulated for individual radionuclides according to source-target region pairs. When source-target regions are individual organs or subregions of organs, fixed geometry models are required to establish geometry, relative orientation, composition, and mass of different source-target organs or organ subregions.