At the time of publication of ICRU Report 32, Methods of Assessment of Absorbed Dose in Clinical Use of Radionuclides (ICRU, 1979), nuclear medicine imaging was in its infancy. Rectilinear scanners were still in use and scintillation cameras were emerging as the standard of practice. Planar techniques were principally used to acquire diagnostic images for ascertaining gross functionality of major organs in the body. To ensure the safety of radiopharmaceuticals used in nuclear medicine, the Medical Internal Radiation Dose (MIRD) Committee of the Society of Nuclear Medicine developed a general formalism for calculation of the absorbed dose from internal radionuclides (Loevinger and Berman, 1976). This formalism, known as the MIRD schema, was adopted by the ICRU in Report 32. At the time, the schema was applied to absorbed-dose calculations using an anthropomorphic phantom representing a 70 kg man (Snyder et al., 1975). These calculations required numerous assumptions (detailed in Section 3.5) to arrive at estimates of mean, organ absorbed doses, which were, in turn, used in risk estimation. The procedures and assumptions recommended in ICRU Report 32 were appropriate with respect to the context of the field of nuclear medicine at that time. Over the years, the development of highly selective radiopharmaceuticals, major improvements in planar imaging, and the acquisition of three-dimensional images with SPECT and PET have spearheaded the transformation of organ based imaging to the acquisition of physiological data on subregions and specific cell populations within organs. Examples of current studies can be seen in a special issue of The Journal of Nuclear Medicine on nuclear neurology (Goldsmith, 1996). Accordingly, the MIRD schema has been adopted to calculate mean absorbed doses using models of suborgan, multicellular, and cellular source and target regions (Coffey and Watson, 1981; Goddu et al., 1994a, 1997; Bouchet et al., 1999). In addition, since the publication ofICRU Report 32, several new features of the MIRD schema have achieved widespread acceptance in the field of dosimetry. Among these