Skeletal Reference Models for Pediatric Patients

D. Hasenauer; C. Watchman; A. Shah; and W. Bolch (University of Florida)

Accurate predictions of the radiation dose to bone marrow are important in medical radiation protection. Current skeletal reference models use chord-length distributions that do not account for energy loss to cortical bone or particle escape into soft tissue at high electron energies. Recently, Shah et al. introduced the PIRT (Paired-Image Radiation Transport) model for skeletal dosimetry which accounts for both the trabecular microstructure and the 3D shape and dimension of the adult skeletal site. In this study, we extend this approach to the pediatric skeleton. Extremely limited data exist on the bone microstructure of children, and thus we have adopted one of the data sets: chord-length distributions for a 1.7-year and 9-year male acquired at the University of Leeds in the early 1970s. These distributions were used as input to CBIST (chord-based infinite spongiosa transport) models of electrons and beta-particles. For the bone macrostructure, we have rescaled each ex-vivo CT image of the UF reference male skeletal model using dimensions given by the UF series of pediatric tomographic models: 9-mo male, 4-year female, 8-year female, 11-year male, and 14-year male. These models were then used in SIRT (Single-Image Radiation Transport) simulations of electron energy escape from spongiosa tissues at each bone site and subject age. The CBIST and SIRT results were then combined to obtain an overall indication of skeletal tissue absorbed fractions as a function of bone site and age. The resulting CBIST-SIRT values of AF clearly indicate that as the size of bone decreases, existing skeletal reference models for pediatric individuals (solely based on CBIST transport methods) increasingly overestimate the absorbed dose to active bone marrow and trabecular endosteum by not accounting for particle escape to the bone cortex. Future efforts should be devoted to 3D microimaging of pediatric spongiosa, thus permitting PIRT model simulations as has been done in the adult male.

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