New mesh-type phantoms and their dosimetric applications, including emergencies.

Committee 2 of the International Commission on Radiological Protection (ICRP) has constructed mesh-type adult reference computational phantoms by converting the voxel-type ICRP Publication 110 adult reference computational phantoms to a high-quality mesh format, and adding those tissues that were below the image resolution of the voxel phantoms and therefore not included in the Publication 110 phantoms. The new mesh phantoms include all the necessary source and target tissues for effective dose calculations, including the 8-40-µm-thick target layers of the alimentary and respiratory tract organs, thereby obviating the need for supplemental organ-specific stylised models (e.g. respiratory airways, alimentary tract organ walls and stem cell layers, lens of the eye, and skin basal layer). To see the impact of the new mesh-type reference phantoms, dose coefficients for some selected external and internal exposures were calculated and compared with the current reference values in ICRP Publications 116 and 133, which were calculated by employing the Publication 110 phantoms and the supplemental stylised models. The new mesh phantoms were also used to calculate dose coefficients for industrial radiography sources near the body, which can be used to estimate the organ doses of the worker who is accidentally exposed by an industrial radiography source; in these calculations, the mesh phantoms were deformed to reflect the size of the worker, and also to evaluate the effect of posture on dose coefficients.

ICRP dose coefficients: computational development and current status.

Major current efforts within Committee 2 of the International Commission on Radiological Protection (ICRP) involve the development of dose coefficients for inhalation and ingestion of radionuclides, and those for exposure to environmental radiation fields. These efforts build upon changes in radiation and tissue weighting factors (Publication 103), radionuclide decay schemes (Publication 107), computational phantoms of the adult reference male and female (Publication 110), external dose coefficients for adult reference workers for idealised radiation fields (Publication 116), models of radionuclide intake (Publications 66, 100 and 130), and models of radionuclide systemic biokinetics (Publication 130). This paper will review the overall computational framework for both internal and external dose coefficients. For internal exposures, the work entails assessment of organ self-dose and cross-dose from monoenergetic particle emissions (specific absorbed fraction), absorbed dose per nuclear transformation (S value), time-integrated activity of the radionuclide in source tissues (inhalation, ingestion, and systemic biokinetic models), and their numerical combination to yield the organ equivalent dose or effective dose per activity inhaled or ingested. Various challenges are reviewed that were not included in the development of Publication 30 dose coefficients, which were based upon much more simplified biokinetic models and computational phantoms. For external exposures, the computations entail the characterisation of environmental radionuclide distributions, the transport of radiation particles through that environment, and the tracking of energy deposition to the organs of the exposed individual. Progress towards the development of dose coefficients to members of the general public (adolescents, children, infants and fetuses) are also reviewed.

The reference phantoms: voxel vs polygon.

The International Commission on Radiological Protection (ICRP) reference male and female adult phantoms, described in Publication 110, are voxel phantoms based on whole-body computed tomography scans of a male and a female patient, respectively. The voxel in-plane resolution and the slice thickness, of the order of a few millimetres, are insufficient for proper segmentation of smaller tissues such as the lens of the eye, the skin, and the walls of some organs. The calculated doses for these tissues therefore present some limitations, particularly for weakly penetrating radiation. Similarly, the Publication 110 phantoms cannot represent 8-40-µm-thick target regions in respiratory or alimentary tract organs. Separate stylised models have been used to represent these tissues for calculation of the ICRP reference dose coefficients (DCs). ICRP Committee 2 recently initiated a research project, the ultimate goal of which is to convert the Publication 110 phantoms to a high-quality polygon-mesh (PM) format, including all source and target regions, even those of the 8-40-µm-thick alimentary and respiratory tract organs. It is expected that the converted phantoms would lead to the same or very similar DCs as the Publication 110 reference phantoms for penetrating radiation and, at the same time, provide more accurate DCs for weakly penetrating radiation and small tissues. Additionally, the reference phantoms in the PM format would be easily deformable and, as such, could serve as a starting point to create phantoms of various postures for use, for example, in accidental dose calculations. This paper will discuss the current progress of the phantom conversion project and its significance for ICRP DC calculations.

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

Abstract ­: Dose coefficients for assessment of internal exposures to radionuclides are radiological protection quantities giving either the organ equivalent dose or effective dose per intake of radionuclide following ingestion or inhalation. In the International Commission on Radiological Protection's (ICRP) Occupational Intakes of Radionuclides (OIR) publication series, new biokinetic models for distribution of internalised radionuclides in the human body are presented as needed for establishing time-integrated activity within organs of deposition (source regions). This series of publications replaces Publications 30 and 68 (ICRP, 1979, 1980, 1981, 1988, 1994b). In addition, other fundamental data needed for computation of the dose coefficients are radionuclide decay data (energies and yields of emitted radiations), which are given in Publication 107 (ICRP, 2008), and specific absorbed fraction (SAF) values ­ defined as the fraction of the particle energy emitted in a source tissue region that is deposited in a target tissue region per mass of target tissue. This publication provides the technical basis for SAFs relevant to internalised radionuclide activity in the organs of Reference Adult Male and Reference Adult Female as defined in Publications 89 and 110 (ICRP, 2002, 2009). SAFs are given for uniform distributions of mono-energetic photons, electrons, alpha particles, and fission-spectrum neutrons over a range of relevant energies. Electron SAFs include both collision and radiative components of energy deposition. SAF data are matched to source and target organs of the biokinetic models of the OIR publication series, as well as the Publication 100 (ICRP, 2006) Human Alimentary Tract Model and the Publication 66 (ICRP, 1994a) Human Respiratory Tract Model, the latter as revised within Publication 130 (ICRP, 2015). This publication further outlines the computational methodology and nomenclature for assessment of internal dose in a manner consistent with that used for nuclear medicine applications. Numerical data for particle-specific and energy-dependent SAFs are given in electronic format for numerical coupling to the respiratory tract, alimentary tract, and systemic biokinetic models of the OIR publication series.

Dosimetric models of the eye and lens of the eye and their use in assessing dose coefficients for ocular exposures.

Based upon recent epidemiological studies of ocular exposure, the Main Commission of the International Commission on Radiological Protection (ICRP) in ICRP Publication 118 states that the threshold dose for radiation-induced cataracts is now considered to be approximately 0.5 Gy for both acute and fractionated exposures. Consequently, a reduction was also recommended for the occupational annual equivalent dose to the lens of the eye from 150 mSv to 20 mSv, averaged over defined periods of 5 years. To support ocular dose assessment and optimisation, Committee 2 included Annex F within ICRP Publication 116 . Annex F provides dose coefficients - absorbed dose per particle fluence - for photon, electron, and neutron irradiation of the eye and lens of the eye using two dosimetric models. The first approach uses the reference adult male and female voxel phantoms of ICRP Publication 110. The second approach uses the stylised eye model of Behrens et al., which itself is based on ocular dimensional data given in Charles and Brown. This article will review the data and models of Annex F with particular emphasis on how these models treat tissue regions thought to be associated with stem cells at risk.

Impact on 141Ce, 144Ce, 95Zr, and 90Sr beta emitter dose coefficients of photon and electron SAFs calculated with ICRP/ICRU reference adult voxel computational phantoms.

The current dose coefficients for internal dose assessment of occupationally exposed persons and the general public were derived using the methodology of the International Commission on Radiological Protection (ICRP), which is similar to the Medical Internal Radiation Dose (MIRD)-type methodology. One component of this methodology is the mathematical representation of the human body (so-called MIRD-type phantoms) developed at the Oak Ridge National Laboratory for calculations of photon specific absorbed fractions (SAFs). Concerning the beta emissions, it is assumed in general that they irradiate only the organ where the radionuclide resides, whereas for walled organs, a fixed fraction of the emitted energy is absorbed within the wall. For the active marrow and bone surface targets, absorbed fractions were explicitly provided in ICRP Publication 30. The ICRP Publications 66 and 100 contain further detailed energy-dependent absorbed fraction data for the airways and the segments of the alimentary tract. In the present work, the voxel phantoms representing the reference male and female adults, recently developed at the Helmholtz Zentrum München-German Research Center for Environmental Health (HMGU) in collaboration with the Task Group DOCAL of ICRP Committee 2, were used for the Monte Carlo computation of photon as well as electron SAFs. These voxel phantoms, being constructed from computed tomography (CT) scans of individuals, are more realistic in shape and location of organs in the body than the mathematical phantoms; therefore, they provide photon SAFs that are more precise than those stemming from mathematical phantoms. In addition, electron SAFs for solid and walled organs as well as tissues in the alimentary tract, the respiratory tract, and the skeleton were calculated with Monte Carlo methods using these phantoms to complement the data of ICRP Publications 66 and 100 that are confined to self-irradiation. The SAFs derived for photons and electrons are then used to calculate the dose coefficients of the beta emitters 141Ce, 144Ce, 95Zr, and 90Sr. It is found that the differences of the dose coefficients due to the revised SAFs are much larger for injection and ingestion than for inhalation. The equivalent doses for colon and ingestion with the new voxel-based SAFs are significantly smaller than the values with the MIRD-type photon SAFs and simplifying assumptions for electrons. For lungs and inhalation, no significant difference was observed for the equivalent doses, whereas for injection and ingestion, an increase of the new values is observed.

New calculations for internal dosimetry of beta-emitting radiopharmaceuticals.

The calculation of absorbed dose from internally incorporated radionuclides is based on the so-called specific absorbed fractions (SAFs) which represent the fraction of energy emitted in a given source region that is absorbed per unit mass in a specific target organ. Until recently, photon SAFs were calculated using MIRD-type mathematical phantoms. For electrons, the energy released was assumed to be absorbed locally ('ICRP 30 approach'). For this work, photon and electron SAFs were derived with Monte Carlo simulations in the new male voxel-based reference computational phantom adopted by the ICRP and ICRU. The present results show that the assumption of electrons being locally absorbed is not always true at energies above 300-500 keV. For source/target organ pairs in close vicinity, high-energy electrons escaping from the source organ may result in cross-fire electron SAFs in the same order of magnitude as those from photons. Examples of organ absorbed doses per unit activity are given for (18)F-choline and (123)I-iodide. The impact of the new electron SAFs used for absorbed dose calculations compared with the previously used assumptions was found to be small. The organ dose coefficients for the two approaches differ by not more than 6 % for most organs. Only for irradiation of the urinary bladder wall by activity in the contents, the ICRP 30 approach presents an overestimation of approximately 40-50%.

SEECAL utilizing voxel-based SAFs.

The computer program SEECAL, written by Cristy and Eckerman from the Oak Ridge National Laboratory, USA, calculates specific effective energies (also known as S factors in the MIRD terminology) for an adult male, an adult female and five paediatric ages. Its dosimetric methodology is that of the ICRP. Among other parameters, SEECAL requires input data on specific absorbed fractions (SAF) and utilises those derived from the MIRD-type stylised anthropomorphic phantoms. SEECAL has been used worldwide for dose estimations concerning occupational or public exposures due to radionuclides incorporated into the body and has formed the basis for programs developed by other laboratories to calculate, for example, dose to the patients undergoing nuclear medicine procedures. The revised version of SEECAL is at the moment limited to adults and utilises the photon SAFs derived with Monte Carlo methods for the new reference male and female voxel-based phantoms to be adopted by the ICRP.

Skeletal absorbed fractions for electrons in the adult male: considerations of a revised 50-microm definition of the bone endosteum.

In 1995, the International Commission on Radiological Protection (ICRP) issued ICRP Publication 70 which provided an extensive update to the physiological and anatomical reference data for the skeleton of adults and children originally issued in ICRP Publication 23. Although ICRP Publication 70 has been a valuable document in the development of reference voxel computational phantoms, additional guidance is needed for dose assessment in the skeletal tissues beyond that given in ICRP Publication 30. In this study, a computed tomography (CT) and micro-CT-based model of the skeletal tissues is presented, which considers (1) a 50-microm depth in marrow for the osteoprogenitor cells, (2) electron escape from trabecular spongiosa to the surrounding cortical bone, (3) cortical bone to trabecular spongiosa cross-fire for electrons and (4) variations in specific absorbed fraction with changes in bone marrow cellularity for electrons. A representative data set is given for electron dosimetry in the craniofacial bones of the adult male.

Organ dose conversion coefficients for voxel models of the reference male and female from idealized photon exposures.

A new series of organ equivalent dose conversion coefficients for whole body external photon exposure is presented for a standardized couple of human voxel models, called Rex and Regina. Irradiations from broad parallel beams in antero-posterior, postero-anterior, left- and right-side lateral directions as well as from a 360 degrees rotational source have been performed numerically by the Monte Carlo transport code EGSnrc. Dose conversion coefficients from an isotropically distributed source were computed, too. The voxel models Rex and Regina originating from real patient CT data comply in body and organ dimensions with the currently valid reference values given by the International Commission on Radiological Protection (ICRP) for the average Caucasian man and woman, respectively. While the equivalent dose conversion coefficients of many organs are in quite good agreement with the reference values of ICRP Publication 74, for some organs and certain geometries the discrepancies amount to 30% or more. Differences between the sexes are of the same order with mostly higher dose conversion coefficients in the smaller female model. However, much smaller deviations from the ICRP values are observed for the resulting effective dose conversion coefficients. With the still valid definition for the effective dose (ICRP Publication 60), the greatest change appears in lateral exposures with a decrease in the new models of at most 9%. However, when the modified definition of the effective dose as suggested by an ICRP draft is applied, the largest deviation from the current reference values is obtained in postero-anterior geometry with a reduction of the effective dose conversion coefficient by at most 12%.

Response functions for computing absorbed dose to skeletal tissues from photon irradiation.

The calculation of absorbed dose in skeletal tissues at radiogenic risk has been a difficult problem because the relevant structures cannot be represented in conventional geometric terms nor can they be visualised in the tomographic image data used to define the computational models of the human body. The active marrow, the tissue of concern in leukaemia induction, is present within the spongiosa regions of trabecular bone, whereas the osteoprogenitor cells at risk for bone cancer induction are considered to be within the soft tissues adjacent to the mineral surfaces. The International Commission on Radiological Protection (ICRP) recommends averaging the absorbed energy over the active marrow within the spongiosa and over the soft tissues within 10 microm of the mineral surface for leukaemia and bone cancer induction, respectively. In its forthcoming recommendation, it is expected that the latter guidance will be changed to include soft tissues within 50 microm of the mineral surfaces. To address the computational problems, the skeleton of the proposed ICRP reference computational phantom has been subdivided to identify those voxels associated with cortical shell, spongiosa and the medullary cavity of the long bones. It is further proposed that the Monte Carlo calculations with these phantoms compute the energy deposition in the skeletal target tissues as the product of the particle fluence in the skeletal subdivisions and applicable fluence-to-dose-response functions. This paper outlines the development of such response functions for photons.

Spectra of scattered photons in large absorbers and their importance for the values of radiation weighting factor wR.

In its review of the present values of radiation weighting factor w(R) and of possible revisions of this factor, the German Radiation Protection Commission has recommended to maintain the approach of ICRP 60 to base the selection of the w(R) value for a given radiation (e.g. fission neutrons) on observed values of the relative biological effectiveness (RBE) of this radiation 'regardless of whether the reference radiation is X rays or gamma rays'. The physical background of the German recommendation is the buildup of a strong field of energy-degraded Compton scattered photons in the human body if exposed to an external field of high-energy photons, so that the total radiation field inside the body is a mixture comprising low and high photon energies. Therefore, it is appropriate that the selection of the w(R) value of the given radiation is guided by RBE values averaged over X rays and gamma rays as the reference radiations. In support of this rationale, the present paper provides a sample of Monte Carlo calculated scattered photon spectra in large absorbers exposed to high-energy photons. Depth-dependent fractional dose contributions of the scattered photons are tabulated for incident energies from 1 to 10 MeV, and estimates of the influence of their degraded energies on the biological effectiveness of the incoming radiation are presented. Accordingly, we point out that it is appropriate to use, for the purposes of 'risk projection', RBE values averaged over X and gamma reference radiations.

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.

The application of voxel phantoms to the internal dosimetry of radionuclides.

Extensive calculations of specific absorbed fractions (SAFs) for monoenergetic photon sources were performed using a Monte Carlo photon transport code together with seven male and female adult voxel models based on computed tomographic data of real persons. These models offer greater realism with respect to organ topology than the mathematical phantoms commonly used in the past. Due to individual anatomical differences, large variations in photon SAFs between the voxel models were found that can amount to orders of magnitude for very low photon energies. However, in many cases, the larger differences were found between MIRD-type and voxel models, since the inter-organ distances tend to be larger in the MIRD-type phantoms than in reality, due to over-simplification of organ shapes. Furthermore, organ absorbed doses per incorporated activity were evaluated for two selected radiopharmaceuticals. Although a method was found to largely eliminate the influence of organ mass on SAFs for organ self-absorption, the absorbed dose coefficients varied by several tens of per cent between the individual voxel models, thus indicating a significant influence of individual photon SAFs for organ cross-fire on organ absorbed dose. Again, 43% of the MIRD organ dose values were outside the range of doses spanned by the voxel models. Effective dose showed a variation of only up to 26% between the single voxel models for the radiopharmaceuticals considered.

Organ dose conversion coefficients for external photon irradiation of male and female voxel models.

New organ equivalent dose conversion coefficients are presented for whole body irradiation with monoenergetic photons of energies between 10 keV and 10 MeV for idealized geometries and seven adult male and female voxel models. The geometries are broad parallel photon beams in anterior-posterior, posterior-anterior, left- and right-lateral direction and a full 360 degree rotation around the body length axis. Dose differences between the different voxel models are below approximately 30% for some organs and geometries in the energy range between 60 and 200 keV, but they can be up to 100% or more in single cases, due to differences in stature and individual anatomical details. For low photon energies, the differences may amount to hundreds of per cent. Extensive comparisons of the dose conversion coefficients with respective values calculated using mathematical body models revealed various degrees of unrealistic positioning of single organs in the latter models. Examples are the kidneys, spleen and stomach that are located too superficially in the mathematical models. Over- or underestimations of several tens of per cent may, thus, occur for the mathematical models, compared to the voxel models considered. In contrast to previous assumptions, when the mathematical models have been used to establish reference organ dose conversion coefficients, it can be concluded that they do not properly represent a large population of individuals.