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Survivor Dosimetry

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Survivor Dosimetry Chapter 12 SURVIVOR DOSIMETRY Part A. Fluence-to-Kerma Conversion Coefficients George D. Kerr, Joseph V. Pace III, Stephen D. Egbert Introduction An important step in the dosimetry evaluation is to relate the radiation passing through a unit volume of a material of interest (fluence) to the energy release (kerma) in the material, which determines the absorbed dose. The fluence-to-kerma conversion coefficients or “kerma coefficients” used in the Dosimetry System 1986 (DS86) are taken from Kerr (1982). These kerma coefficients are based on body tissue compositions for Reference Man from the International Commission on Radiological Protection (1975) and Kerr (1982), the mass energy- absorption coefficients for photons from Hubbell (1982), and the elemental kerma coefficients for neutrons from Caswell et al. (1980). Hence, the kerma coefficients used in DS86 are approximately 20 years old. In order to provide an updated set of kerma coefficients for use in the Dosimetry System 2002 (DS02), a new evaluation has been completed. This new evaluation considered recently suggested changes in the composition of soft tissues of the body in ICRU Report 44 (International Commission on Radiation Units and Measurements 1989), the mass energy- absorption coefficients for photons by Hubbell and Seltzer (1996), and the elemental kerma coefficients for neutrons in ICRU Report 63 (International Commission on Radiation Units and Measurements 2000). The new DS02 kerma coefficients for soft tissue are presented as both point-wise data for use in Monte Carlo radiation transport calculations and multigroup data for use in discrete ordinates radiation transport calculations. Elemental Composition of Soft Tissue Various approximations to the elemental composition of the tissues and organs of the human body have been used in both kerma and dose calculations for photons and neutrons. Some 831 calculations have considered as few as the four major elements of the body (Auxier et al. 1968, Snyder 1972) and others as many as 15 elements (Singh 1982) or more (White and Fitzgerald 1977). The DS86 kerma and dose calculations for both neutrons and photons are based on a twelve-element approximation for the organs and tissues of ICRP-1975 Reference Man. It includes the eleven most abundant elements of the total body (i.e., the skeleton and total soft tissues) and iron, which is the one of the most abundant trace elements in organs and tissues of special interest such as the lungs and bone marrow. A summary of the twelve-element approximation used for total soft tissues of the body in the DS86 kerma and dose calculations is provided in Table 1 (Kerr 1982) White et al. (1987) and the International Commission on Radiation Units and Measurements (ICRU 1989, 1992) recently recommended some changes to the elemental composition of the organs and tissues of ICRP-1975 Reference Man (Table 1). These changes involve only data for elements that contribute more than 0.1% by mass to the composition of any organ or tissue of Reference Man. Thus, the data are limited to the nine most abundant elements in the total soft tissues of the human body, and there appears to be no compelling reason for adopting these suggested revisions to ICRP-1975 Reference Man over those already in use in DS86 (Kerr 1982). For example, the kerma or dose from neutrons depends primarily on the amount of hydrogen in the tissue or organ of interest, and essentially no differences are found among the various hydrogen values of Table 1. The slight differences in hydrogen and other elemental abundances of Table 1 also have a very small impact on the overall mass energy-absorption coefficients for photons in the soft tissues of the body. 832 Kerma Coefficients for Soft Tissue Kerma is the sum of the initial energies of all charged particles liberated by indirectly ionizing radiations such as neutrons and protons in a small volume element of a specified material divided by the mass of material in that volume element (Roesch and Attix 1968). It is a useful quantity in radiation dosimetry when charged particle equilibrium exists at the position and in the material of interest, and bremsstrahlung losses of the charged particles are negligible. In this case, kerma and absorbed dose can be equated. Absorbed dose is the energy imparted by charged particles in the small volume element of the specified material divided by the mass of the material within that volume element. Units of absorbed dose and kerma can be either rad or gray. One rad is equal to 100 erg per gram of the specified material, and 1 gray (Gy) is equal to one joule per kilogram of the specified material (or 100 rad). The kerma coefficients given here are the kerma in soft tissue of the body per unit particle fluence of either neutrons or photons at a specified energy. In many practical applications, the tissue volume of interest may be located in the body or in another medium. For example, the intensity of a radiation field of neutrons and photons incident on the body may be specified in terms of the kerma in air. The tissue kerma in air or so-called free-in-air (FIA) tissue kerma from photons and neutrons is closely related to the maximum absorbed dose in the body (i.e., the maximum absorbed dose to the skin of the body). If the particle fluence involves a broad energy spectrum of neutrons or photons, then an appropriately weighted mean value must be calculated. The mean value would be weighted by the energy spectrum of the particles in air if the quantity of interest is the FIA tissue kerma and by the energy spectrum of the particles in the body if the quantity of interest is the organ dose (i.e., the absorbed dose in the organ). The energy spectrum of the particles within the organ of interest in the body can be calculated using Monte Carlo radiation transport codes. The absorbed dose to critical organs and tissues of the body is the quantity of interest in the radiation dosimetry for the atomic-bomb survivors. Because bremsstrahlung losses by charged particles are negligible in the soft tissues of the body and charged particle equilibrium exists at the interfaces of the soft tissues, absorbed dose and kerma can be equated in most soft tissues of the body once the self shielding by overlying body tissues is taken into account. This approach is used in calculating absorbed doses (or organ doses) for the bladder, brain, breasts, eyes, intestinal tract, kidneys, liver, lungs, ovaries, pancreas, stomach, testes, and thyroid. In the skeleton, the critical tissues are considered to be the red marrow and the osteogenic cells, especially those on the endosteal surfaces of bone. However, absorbed dose and kerma cannot be equated in these tissues because charged particle equilibrium may not exist near a soft tissue-bone interface. The calculation of the absorbed doses to the soft tissue of bone is a more difficult problem which requires that the charged particles produced by the photons and neutrons be tracked in detail as they deposit their energies in the complex intermixture of bone and soft tissues of the skeleton (Kerr and Eckerman 1987). Photon Kerma Coefficients for Soft Tissue Photon kerma coefficients for soft tissue are obtained by summing the products of the mass faction on an element in soft tissue, the photon energy, and the mass energy-absorption coefficient of the element for photons of that energy. The sums are calculated for discrete photon 833 energies, and the kerma coefficients are referred to as “point-wise data.” If the unit of photon energy is MeV, and the units of the mass energy-absorption coefficients are cm2 per g, then the sums can be multiplied by 1.602 × 10-10 to obtain kerma coefficients with units of Gy per photon per cm2. The mass factions for soft tissue were taken from the 1982 report by Kerr (see column 2 of Table 1) and the mass energy-absorption coefficients were taken from the report by Hubbell and Seltzer (1996). The newer kerma coefficients (DS02) and the older kerma coefficients (DS86) for photons in soft tissue of ICRP-1975 Reference Man are compared in Table 2. The only noticeable departure between the two sets of kerma coefficients occurs at energies above 10 MeV, with the older DS86 kerma coefficients being larger than the newer DS02 kerma coefficients. The maximum difference between the two sets of kerma coefficients occurs at 20 MeV and is approximately 7%. Neutron Kerma Coefficients for Soft Tissue The DS86 studies used kerma coefficients for neutrons in 19 different isotopes and elements, including the 12 elements used in the soft tissue-approximation for ICRP-Reference Man, that were tabulated by Caswell et al. (1980). Their tabulations gave the kerma coefficients for a monoenergetic “thermal neutron” energy of 0.0235 eV and for 119 contiguous energy “groups” or “bins” extending from 0.026 eV to 30 MeV. Each bin was characterized by a central or mean energy and an energy interval of a given width (Caswell et al. 1980). The kerma coefficients were calculated from cross sections averaged over the full energy width of each bin. Averaging over binned energy widths eliminated the somewhat irregular behavior of the kerma coefficients due to resonance absorption of neutrons by elements other than hydrogen. Only the tabulated data for bins with neutron energies below 20 MeV were used in the DS86 studies. 834 The neutron kerma coefficients of Caswell et al. (1980) have been revised for 12 different isotopes and published in Report 63 of the International Commission on Radiation Units and Measurements (2000). Because these revisions are more limited than before (Caswell et al.
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