S values for radionuclides localized within the skeleton

Calculations of radiation absorbed dose to the active marrow are important to radionuclide therapies such as radioimmunotherapy and bone pain palliati on. In diagnostic nuclear medicine, calculations of the effective dose for radiopharmaceutical procedures also require the assessment of radiation dos e to the skeletal endosteum. We have previously reported the development of 2 3-dimensional electron transport models for assessing absorbed fractions to both marrow and endosteum in trabecular and cortical bone, respectively . Here, we extend these calculations to the assignment of radionuclide S va lues. Methods: Data published in International Commission on Radiological P rotection Publication 70 were used to develop tables of masses for total ma rrow space, active and inactive marrow, endosteum, and bone matrix within 2 2 skeletal sites in the adult. Using our sif:e-specific tissue masses, alon g with electron absorbed fractions given by our 3-dimensional transport mod els, radionuclide S values (electron and beta particle components only) wer e subsequently calculated using the MIRD schema for P-32, P-33, Sr-89, Sr-9 0, Y-90, Sn-117m, Sm-153, Er-169, Lu-177, and Re-186. Specific consideratio n was given to the trabecular active marrow as both a source and a target r egion. Results: Site-specific radionuclide S values are reported for 22 ske letal sites, for 9 source-target tissue combinations within trabecular bone , and for 6 source-target tissue combinations within cortical bone. Skeleta l-averaged S values are also provided. Conclusion: A fully documented model is presented for the adult for use in radionuclide dosimetry of the skelet on. The model is based on both the latest international recommendations for skeletal tissue masses and results from three-dimensional electron transpo rt calculations within the skeleton. Comparisons are additionally made agai nst the radionuclide S values published in MIRD Pamphlet No. 11 and those c alculated using the MIRDOSE2 and MIRDOSE3 computer codes. Differences in th ese datasets vary with the sou me-target combination considered and may be attributed to 1 of 3 causes: (a) assumptions on reference target masses, (b ) transport models used to assign absorbed fractions, and (c) implicit assu mptions made in considering the trabecular active marrow as both a source a nd a target tissue.