Response functions for computing absorbed dose to skeletal tissues from neutron irradiation

Abstract

Spongiosa in the adult human skeleton consists of three tissues - active marrow (AM), inactive marrow (IM), and trabecularized mineral bone (TB). Active marrow is considered to be the target tissue for assessment of both long-term leukemia risk and acute marrow toxicity following radiation exposure. The total shallow marrow (TM50), defined as all tissues laying within the first 50 μm the bone surfaces, is considered to be the radiation target tissue of relevance for radiogenic bone cancer induction. For irradiation by sources external to the body, kerma to homogeneous spongiosa has been used as a surrogate for absorbed dose to both of these tissues, as direct dose calculations are not possible using computational phantoms with homogenized spongiosa. Recent microCT imaging of a 40-year-old male cadaver has allowed for the accurate modeling of the fine microscopic structure of spongiosa in many regions of the adult skeleton [Hough et al PMB (2011)]. This microstructure, along with associated masses and tissue compositions, was used to compute specific absorbed fractions (SAF) values for protons originating in axial and appendicular bone sites [Jokisch et al PMB (submitted)]. These proton SAFs, bone masses, tissue compositions, and proton production cross-sections, were subsequently used to construct neutron dose response functions (DRFs) for both AM and TM50 targets in each bone of the reference adult male. Kerma conditions were assumed for other resultant charged particles. For comparison, active marrow, total shallow marrow, and spongiosa kerma coefficients were also calculated. At low incident neutron energies, AM kerma coefficients for neutrons correlate well with values of the AM DRF, while total marrow (TM) kerma coefficients correlate well with values of the TM50 DRF. At high incident neutron energies, all kerma coefficients and DRFs tend to converge as charged particle equilibrium (CPE) is established across the bone site. In the range of 10 eV to 100 MeV, substantial differences are observed among the kerma coefficients and DRF. As a result, it is recommended that the AM kerma coefficient be used to estimate the AM DRF, and that the TM kerma coefficient be used to estimate the TM50 DRF below 10 eV. Between 10 eV and 100 MeV, the appropriate DRF should be used as presented in this study. Above 100 MeV, spongiosa kerma coefficients apply well for estimating skeletal tissue doses. DRF values for each bone site as a function of energy are provided in an electronic annex to this article.

Conversion Coefficients for Radiological Protection Quantities for External Radiation Exposures

“…In contrast, elastic and inelastic collisions of neutrons at energies below $150 MeV in spongiosa will result in a greater number of recoil protons born within the marrow tissues than in the bone trabeculae, due to the higher hydrogen content in the former, and these recoil particles will, in many cases, traverse the marrow spaces with their residual energy being lost to surrounding trabeculae. The net result for neutron irradiation over many energy regions is then a suppression of the absorbed dose to marrow tissues in comparison with that predicted in a kerma approximation (Kerr and Eckerman, 1985;Bahadori et al, 2011).…”

“…(E3) In this report, Eq. (E.2) was evaluated using methods given in Bahadori et al (2011). Proton absorbed fraction data were obtained through Continuous Slowing Down Approximation (CSDA) transport methods using linear pathlength distributions acquired from micro-computer-tomography images of 32 bone sites within the skeleton of a 40-year-old male cadaver (Jokisch et al, 2011a,b).…”

ICRP Publication 130: Occupational Intakes of Radionuclides: Part 1

“…Values of absorbed fractions to active marrow and endosteum for internally-emitted photons and neutrons were obtained by first tallying energy-dependent particle fluences within the spongiosa and medullary cavity regions of the Publication 110 Reference Adult Male and Reference Adult Female voxel phantoms (ICRP, 2009), and then applying fluence-to-absorbed dose response functions (DRFs). Further details on the derivation of these photon and neutron skeletal DRFs are given in Johnson et al (2011) and Bahadori et al (2011), respectively, as well as in Annexes D and E of Publication 116 (ICRP, 2010).…”

“…Further details on the derivation of these photon and neutron skeletal DRFs are given in Johnson et al. (2011) and Bahadori et al. (2011), respectively, as well as in Annexes D and E of Publication 116 (ICRP, 2010).…”

ICRP Publication 133: The ICRP computational framework for internal dose assessment for reference adults: specific absorbed fractions

“…The energy deposition in these tissues is influenced by their proximity to those of different densities and elemental compositions. This energy deposition can, however, be derived during the Monte Carlo calculations of photon transport through scaling the calculated photon fluence in different skeletal regions (spongiosa or medullary cavities) by functions representing the absorbed dose to the target tissue per photon (Eckerman, 1985; Eckerman et al., 2008; Johnson et al., 2011) or neutron fluence (Bahadori et al., 2011). These functions, referred to as ‘response functions R’, are derived using models of the microscopic structure of bone geometry of different skeletal regions and the transport of the secondary ionising radiations through those geometries.…”