MIRD Pamphlet No. 19: Absorbed Fractions and Radionuclide S Values for Six Age-Dependent Multiregion Models of the Kidney

As one of the major organs of the excretory pathway, the kidneys represent a frequent source of radiopharmaceutical uptake in both diagnostic and therapeutic nuclear medicine. The unique organization of the functional tissues of the organ ensures transient changes in suborgan localization of renal activity. Current single-region dosimetric models of the kidneys, however, force the assumption of a uniform distribution of radioactivity across the entire organ. The average absorbed dose to the kidneys predicted by such models can misrepresent local regional doses to specific substructures. To facilitate suborgan dosimetry for the kidneys, 6 new age-dependent multiregion kidney models are presented. The outer dimensions of the models conform to those used currently in single-region kidney models, whereas interior structures are defined for the renal cortex, the medullary pyramids with papillae (2 vertical and 3 horizontal), and the renal pelvis. Absorbed fractions of energy were calculated for both photon and electron sources (10 keV to 4 MeV) located in each source region within the 6 age-dependent models. The absorbed fractions were then used to assemble S values for radionuclides of potential interest in suborgan kidney dosimetry. For the adult, the absorbed dose to the renal cortex for (90)Y-labeled compounds retained within that subregion is approximately 1.3 times that predicted by the single-region kidney model, whereas the medullary dose is only 26% of that same single-region value. For compounds that are rapidly filtered in the kidneys, the renal cortex dose is approximately one-half of that predicted under the single-region model, whereas the tissues of the medullary pyramids receive an absorbed dose 1.5-1.8 times larger. The multiregion model described here permits estimates of regional kidney dose not previously supported by current single-region models. Full utilization of the new model, however, requires serial imaging of the kidneys with regions of interest assigned to the renal cortex and medulla.