Fluence-to-absorbed dose conversion coefficients for use in radiological protection of embryo and foetus against external exposure to electrons from 10 MeV TO 10 GeV

Electrons as primary and more often as secondary radiation exist commonly in the environment and workplaces. No conversion coefficients are yet available, in the literature, for use in radiological protection of embryo and foetus against external exposure to electrons. This study uses mathematical models developed by the Radiation Protection Bureau, Health Canada, for the embryo of 8 wk and for the foetus of 3, 6, and 9 mo. Monte Carlo code MCNPX is used to determine mean absorbed doses to the embryo and foetus when the mother is exposed to external electron fields. Monoenergetic electrons ranging from 10 MeV to 10 GeV were considered. The irradiation geometries include antero-posterior (AP), postero-anterior (PA), lateral (LAT), rotational (ROT), isotropic (ISO), and top-down (TOP). At each of these irradiation geometries, absorbed doses to the foetal brain and body were calculated for the embryo of 8 wk and the foetus of 3, 6, and 9 mo. Electron fluence-to-absorbed dose conversion coefficients were derived for the four prenatal ages.     

Mathematical models of the embryo and fetus for use in radiological protection

This development of new mathematical models arose from our current work in external neutron dosimetry for the embryo and fetus when pregnant women travel at commercial aircraft altitudes. A problem of concern in radiation protection is exposure of pregnant women to ionizing radiation because of the high radiosensitivity of the embryo and fetus. Special regulations and dosimetric considerations are necessary for pregnant women at the work place and in the public. To perform dosimetry, mathematical models for the embryo and the fetus, together with the modified adult female model for pregnant woman, are required. There are no models available for embryo. Models developed for the fetus need to be updated with the new reference values such as those in ICRP Publication 89. This article presents mathematical models for the embryo and fetus at different stages: the embryo at 8 wk and the fetus at the end of each trimester. In addition to fetal skeleton, the fetal brain is explicitly modeled because of its high radiosensitivity. All model parameters are determined from the most recent reference values available. The models are designed so that an interpolation can be easily performed to generate a model of embryo/fetus at any given stage of development. This feature also allows convenient adaptation of the models to different reference values representing various ethnic populations. The new mathematical models presented here were developed for external dosimetry. They can also be used for internal dosimetry purposes, if other organs inside the female phantom are adjusted accordingly.     

Fluence-to-dose conversion coefficients based on the VIP-Man anatomical model and MCNPX code for monoenergetic neutrons above 20 MeV

A new set of fluence-to-absorbed dose and fluence-to-effective dose conversion coefficients has been calculated for high-energy neutrons using a whole-body anatomical model, VIP-Man, developed from the high-resolution transversal color photographic images of the National Library of Medicine's Visible Human Project. Organ dose calculations were performed using the Monte Carlo code MCNPX for 20 monoenergetic neutron beams between 20 MeV and 10,000 MeV under 6 different irradiation geometries: anterior-posterior, posterior-anterior, left lateral, right lateral, isotropic, and rotational. For neutron Monte Carlo calculations, results based on an image-based whole-body model were not available in the literature. The absorbed dose results for 24 major organs of VIP-Man are presented in the form of tables and selected figures that compare with those based on simplified mathematical phantoms reported in the literature. VIP-Man yields up to 40% larger values of effective dose and many organ doses, thus suggesting that the results reported in the past may not be conservative.     

Organ dose conversion coefficients for 0.1-10 MeV electrons calculated for the VIP-Man tomographic model

A whole-body tomographic model, called VIP-Man, was recently developed at Rensselaer Polytechnic Institute from the high-resolution color photographic images of the National Library of Medicine's Visible Human Project. An EGS4-based Monte Carlo user code, named EGS4-VLSI, was developed to efficiently transport electrons using the large image data set for VIP-Man. VIP-Man has been used to calculate doses for neutrons and photons. This paper presents a new set of fluence-to-absorbed-dose conversion coefficients for monoenergetic electron beams between 100 keV and 10 MeV for VIP-Man. Irradiation conditions include anterior-posterior, posterior-anterior, right lateral, left lateral, rotational, and isotropic source geometries. Comparisons between organ doses from VIP-Man, which is taller and heavier than the Reference Man, and existing data from mathematical models show significant discrepancies. It appears that even slight differences between body models can cause dramatic dosimetric deviations for low penetrating electron irradiation. This suggests that a single standard body model may poorly represent a large population and may not be acceptable for electron dosimetry.     

Adult female voxel models of different stature and photon conversion coefficients for radiation protection

This paper describes the construction of three adult female voxel models, two whole-body and one from head to thighs, from computed tomographic data of 3 women of different stature. Voxel models (also called phantoms) are human models based on computed tomographic or magnetic resonance images obtained from high resolution continuous scans of a single individual. The gray-scale data or information content of the medical images are interpreted into tissues (i.e., organs), a process known as segmentation. The phantoms, consisting of millions of volume elements, called voxels, provide a three-dimensional representation of the human body and the spatial form of its constituent organs and structures. They were initially developed for radiation protection purposes to estimate the organ and effective doses and hence the risk to a person or population due to an irradiation. This paper also presents conversion coefficients for idealized geometries of external photon exposures of energies 10 keV-1 MeV for the three female models, calculated with a Monte Carlo code. Until now there were not any published data on conversion coefficients for explicit female voxel models. Such sets of conversion coefficients exist for voxel adult males or for MIRD-type male, female, and hermaphrodite models. Numerical differences of the calculated conversion coefficients for the voxel female models and MIRD-type models can amount up to 60% or more for external exposures and are due to the improved anatomical realism of the voxel models. The size of the model also has an effect on the conversion coefficients, particularly for deeper lying organs and energies below 200 keV. The three separate sets of conversion coefficients allow one to choose the most suitable model according to the size of the individual as well as to study the dosimetric variations due to the size of the model.     

Fluence to dose equivalent conversion factors calculated with EGS3 for electrons from 100 keV to 20 GeV and photons from 11 keV to 20 GeV

The EGS3 Monte-Carlo electron-photon transport simulation package has been used to calculate dose equivalent per unit fluence vs depth curves for broad parallel beams of mono-energetic electrons, positrons and photons incident on a 30-cm-thick slab of ICRU four-element tissue. The electron kinetic energy range covered is 100 keV to 20 GeV and that for photons is 11 keV to 20 GeV. It was found that by making minor modifications, EGS3 is in reasonable agreement with other codes for electron energies down to 100 keV. Complete dose equivalent vs depth curves as a function of electron and photon energy are presented to allow proper calculations of the maximum dose equivalent for a mixed photon and electron spectrum since there are substantial variations in the locations of the peak dose equivalent. Explicit calculations demonstrate that l/r2 corrections give an accurate means to convert results for broad parallel beams to those for point source geometries. The relative contributions of various physical processes to the peak dose equivalent are presented.     

Estimates of absorption of radiofrequency radiation by the embryo and fetus during pregnancy.

This paper reports that the specific absorption rate induced in the embryo or fetus can exceed that recommended for the general public when the mother is exposed to radiofrequency radiation at the occupational limits. This result applies to two-tiered radiofrequency radiation standards where a factor of 5 differentiates occupational and nonoccupational exposure limits. Using simple axisymmetric geometries for the pregnant worker, and assuming plane wave exposures, a finite element method provides estimates of prenatal exposure. Various layered shapes are used to model skin, fat, uterus, blood, embryonic, and fetal tissues. Applying current exposure limits given by IRPA, ANSI, and SAA, the results indicate that overexposures to the embryo or fetus can occur from early pregnancy at 80-100 MHz, and in late pregnancy across the range 300-1500 MHz.     

Determination and use of the monetary values of the averted person-sievert for use in radiation protection decisions in Hungary

The monetary value of the averted dose is a key element in the implementation of the optimization principle both in radiation praxis and intervention. The main concept of this principle is to select options so as to maintain exposures at a reasonable level. The feature of this concept is to look for the minimal total cost, i.e., the sum of the costs of protection and health detriment. In its publications, ICRP emphasized the need for developing models which also take into account the "subjective" aspects of health detriment in the optimization process, such as the perception of risk by individuals and the need to put more emphasis on equity in the distribution of individual doses. This paper proposes a modified alpha-value model based on CEPN's model (Centre d'Etude sur L'Evaluation de la Protection dans le Domaine Nucleaire) to put more emphasis on recently published considerations about the smaller effects of the portion of collective dose derived from small doses. The parameters of the monetary value of unit collective dose averted, which is a key element of this type of model, can be estimated by means of approaches like human capital (HC) and willingness to pay (WTP) from the point of view of economic theories. The present study summarizes the results achieved by WTP among the radiation specialists mainly from the Paks Nuclear Power Plant, Hungary. The aim of the effort was to determine the value of a statistical life and the monetary value of a unit person-sievert associated with averted occupational exposure due to ionizing radiation. To apply the WTP method, a questionnaire has been prepared on the basis of the one introduced by CEPN in the late 1990's. The investigations show that the value of US$6,200 person-Sv(-1) seems to be acceptable for the alphabase-value for the occupational situation in Hungary in 1999. WTP assessments should be applied with caution since the economic level of the country, the workplace surveyed, and the computational methods affect the results. In addition, achieving a high level safety culture must rely on international cooperation both from the theoretical and practical viewpoints, and international markets affect the associated costs. Therefore the monetary requirements cannot always be assessed solely on a national basis.     

Prospects for new information relevant to radiation protection from studies of experimental animals

The theory underlying radiation protection was developed from studies of people, laboratory animals, tissues, cells and macromolecules. Data on people were obtained from opportunistic studies of individuals previously exposed to radiation. Rarely has it been possible to conduct prospective studies of people exposed to known quantities of radiation, which sharply restricts the nature of questions that they can address. In contrast, studies using laboratory animals and simpler biological systems can be designed to address specific questions, using controlled exposure conditions. In-vitro research with macromolecules, cells and tissues leads to understanding normal and disease processes in isolated biological components. Studies of the intact animals provide opportunities to study in vivo interactive mechanisms observed in vitro and their role in development of radiation-induced diseases such as cancer. In the future, studies of intact animals should prove increasingly valuable in linking new knowledge at the subanimal level with the more fragmentary information obtained from direct observations on people. This will provide insight into important issues such as (a) effects of low-level radiation exposures, (b) mechanism of cancer induction at high versus low radiation doses, and (c) influence of factors such as nutrition and exposure to chemicals on radiation-induced cancer. This presentation describes strategies for conducting and integrating results of research using macromolecules, cells, tissues, laboratory animals and people to improve our understanding of radiation-induced cancer. It will also emphasize the problems encountered in studies at all levels of biological organization when the disease is observed in low "excess incidence" long after exposure to the toxicant.     

Results of 10 y of radiation protection dosimetry at the neutrontherapy facility in Louvain-la-Neuve

After 10 y of routine operation, radiation hazards to the neutron therapy staff at Louvain-la-Neuve were evaluated. These hazards to the staff in neutron therapy are a matter of concern, not only because of the dose levels due to induced activation after treatment but also because of the difficulty of evaluating adequately the dose equivalent rates (including the quality factor, QF) during the irradiation. Build-up of radioactivity near the collimator and in the therapy room is reported. In the beam axis, dose rates due to activation can amount up to 5 mGy h-1. However, these rates are efficiently reduced to 0.3 mGy h-1 by automatically withdrawing the target after irradiation. Other problems of radioactive contamination (for instance, the choice of the Fe composition of the collimator) are discussed. Dose equivalent rates were determined during treatment at different positions inside and outside the treatment room. At these locations, neutrons of widely varying energies are present, accompanied by a significant proportion of gamma rays. The measurements of neutron-dose equivalent rates were performed with three commercial neutron detectors: a BF3 and a He3 remcounter (Nuclear Enterprises and Nardeux) and the Dineutron multisphere remcounter (Nardeux). The Dineutron also allowed the evaluation of QF. The results of these detectors were compared with the results of the integration of microdosimetry spectra obtained in free air with a TE proportional counter (Far West Technology). For the p(65) + Be neutrons, the dose equivalent rate was equal to 0.4 Sv h-1 in the treatment room (at 2.5 m from the collimator) and was reduced to 2-3 microSv h-1 at the room entrance. The gamma component increased from 55% to 95% at the same positions, respectively. Ratios of the responses of all detectors and QF values are compared and discussed at the different positions. The whole-body dose equivalents to the neutron therapy staff over the 10 y considered are presented; although they are higher than for conventional radiotherapy, they remain far below the ICRP recommended dose limits.     

A technical basis for the continued use of exposure as an acceptable operational quantity in radiation protection

Within the tabulated values of the new [to U.S. Department of Energy (DOE)] radiation weighting factors, it can be seen that a doubling of the neutron factor occurs for the 0.1 to 2 MeV neutron energy range. Hence, with the effective replacement of the quality factor by these new radiation weighting factors (for the protection quantities), it has been widely understood that the new changes will most definitely impact neutron dosimetry. However, it is less well understood that the new changes could also affect photon (and beta) dosimetry, i.e., photon reference fields, instrument design, and instrument calibrations. This paper discusses the ramifications, and ultimately concludes that the use of exposure for workplace measurements complies with both current and amended DOE requirements.