An image-based skeletal dosimetry model for the ICRP reference adult female—internal electron sources

Abstract

An image-based skeletal dosimetry model for internal electron sources was created for the ICRP-defined reference adult female. Many previous skeletal dosimetry models, which are still employed in commonly used internal dosimetry software, do not properly account for electron escape from trabecular spongiosa, electron cross-fire from cortical bone, and the impact of marrow cellularity on active marrow self-irradiation. Furthermore, these existing models do not employ the current ICRP definition of a 50 µm bone endosteum (or shallow marrow). Each of these limitations was addressed in the present study. Electron transport was completed to determine specific absorbed fractions to both active and shallow marrow of the skeletal regions of the University of Florida reference adult female. The skeletal macrostructure and microstructure were modeled separately. The bone macrostructure was based on the whole-body hybrid computational phantom of the UF series of reference models, while the bone microstructure was derived from microCT images of skeletal region samples taken from a 45 years-old female cadaver. The active and shallow marrow are typically adopted as surrogate tissue regions for the hematopoietic stem cells and osteoprogenitor cells, respectively. Source tissues included active marrow, inactive marrow, trabecular bone volume, trabecular bone surfaces, cortical bone volume, and cortical bone surfaces. Marrow cellularity was varied from 10 to 100 percent for active marrow self-irradiation. All other sources were run at the defined ICRP Publication 70 cellularity for each bone site. A total of 33 discrete electron energies, ranging from 1 keV to 10 MeV, were either simulated or analytically modeled. The method of combining skeletal macrostructure and microstructure absorbed fractions assessed using MCNPX electron transport was found to yield results similar to those determined with the PIRT model applied to the UF adult male skeletal dosimetry model. Calculated skeletal averaged absorbed fractions for each source-target combination were found to follow similar trends of more recent dosimetry models (image-based models) but did not follow results from skeletal models based upon assumptions of an infinite expanse of trabecular spongiosa.

Quantitative impact of changes in marrow cellularity, skeletal size, and bone mineral density on active marrow dosimetry based upon a reference model

“…These relative changes correspond to major changes in absolute bone trabeculae volume for the parietal bone whose reference TBVF is 68%. This is in contrast to smaller absolute changes in trabecular bone volume for the lower ribs and L3 lumbar vertebrae whose reference TBVFs are only 9.1% and 12.6%, respectively . As a result, changes in values of S ( AM ← AM , TBV , TBS ) for the latter two bone sites are generally only a few percent of reference S values.…”

“…In 2016, O'Reilly created a comprehensive electron skeletal model of the reference adult female as defined in ICRP Publications 70 and 89 . This model follows the study of Hough et al .…”

“…Bone macrostructural models were taken directly from the individual bones within the University of Florida (UF) hybrid computational phantom of the reference adult female . In the present study, the bone models of the lower ribs, L3 lumbar vertebra, and parietal bone of the cranium are taken from the O'Reilly model of the reference adult female . These particular bone sites were chosen based upon two criteria: (1) relative high fractions of total bone active bone marrow, and (2) a wide variation in the bone trabecular microarchitecture.…”

Depth‐dependent concentrations of hematopoietic stem cells in the adult skeleton: Implications for active marrow dosimetry

“…The corresponding microstructures for these bone sites were taken from some 37 microCT images of cored spongiosa reported by O'Reilly . For many of the bone sites, only a single representative sample was taken (e.g., iliac crest to represent the os coxae).…”

“…In 2016, O'Reilly created a comprehensive electron skeletal model of the reference adult female as defined in ICRP Publications 70 and 89 . This model followed that by Hough et al.…”