Investigations at the University of Leeds under the direction of F.W. Spiers in the early 1960s through the late 1970s established the first comprehensive assessment of marrow dose conversion factors (DCFs) for beta-emitting radionuclides within the volume or on the surface of trabecular bone. These DCFs were subsequently used in deriving radionuclide S values for skeletal tissues published in MIRD Pamphlet No. 11. Eckerman re-evaluated this work and extended the methods of Spiers to radionuclides within the marrow to provide DCFs for fifteen skeletal regions in computational models representing individuals of six different ages. These results were used in the MIRDOSE3 software. Bouchet et al. used updated information on regional bone and marrow masses, as well as 3D electron transport techniques, to derive radionuclide S values in skeletal regions of the adult. Although these two efforts are similar in most regards, the models differ in three respects in: (1) the definition of the red marrow region, (2) the definition of a surface source of activity, and (3) the assumption applied in transporting electrons through the trabecular endosteum. In this study, a review of chord-based skeletal models is given, followed by a description of the differences in the Eckerman and Bouchet et al. transport models. Finally, new data from NMR microscopy and radiation transport in trabecular bone is applied to address item (1) above. Dose conversion factors from MIRD 11, the Eckerman model, the Bouchet et al. model, and a revised model are compared for several radionuclides important to internal emitter therapy.