Computational Phantoms of the ICRP Reference Male and Reference Female

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1 Computational Phantoms of the ICRP Reference Male and Reference Female Maria Zankl a*, Janine Becker a, Helmut Schlattl a, Nina Petoussi-Henss a, Keith F. Eckerman b, Wesley E. Bolch c, Christoph Hoeschen a a Helmholtz Zentrum München German Research Center for Environmental Health **, Institute of Radiation Protection, Ingolstaedter Landstr., Neuherberg, Germany. b Oak Ridge National Laboratory, Life Science Division, 6 Commerce Park, Oak Ridge, TN , USA. c University of Florida, Departments of Nuclear and Radiological Engineering and Biomedical Engineering, Gainesville, FL , USA. Abstract. Computational models of the human body together with radiation transport codes have been used for the evaluation of organ dose conversion coefficients in occupational, medical and environmental radiation protection. During the last two decades, it has become common practice to use voxel models that are derived mostly from (whole body) medical image data of real persons instead of the older mathematical MIRD-type body models. It was shown that the schematic organ shapes of the MIRD-type phantoms presented an oversimplification, having an influence on the resulting dose coefficients, which may deviate systematically from those calculated for voxel models. In its recent recommendations, the ICRP adopted a couple of voxel phantoms for future calculations of organ dose coefficients. The phantoms are based on medical image data of real persons and are consistent with the information given in ICRP Publication 89 on the reference anatomical and physiological parameters for both male and female subjects. The reference voxel models were constructed by modifying the voxel models "Golem" and "Laura" developed in our working group of two individuals whose body height and weight resembled the reference data. The organ masses of both models were adjusted to the ICRP data on the Reference Male and Reference Female, without spoiling their realistic anatomy. This paper describes the methods used for this process and the characteristics of the resulting voxel models. Furthermore, to illustrate the uses of these phantoms, conversion coefficients for some external exposures are also presented. KEYWORDS: Computational models; voxel phantoms; ICRP Reference Male and Reference Female; organ doses; conversion coefficients.. Introduction Computational phantoms of the human body together with radiation transport codes have been used for the evaluation of organ dose conversion coefficients in occupational, medical and environmental radiation protection. During the last two decades, voxel models were introduced that are derived mostly from (whole body) medical image data of real persons instead of the older mathematical MIRD-type body models. Among other laboratories, the Helmholtz Zentrum München German Research Center for Environmental Health (i.e., the former GSF National Research Center for Environment and Health) has developed 2 voxel phantoms of individuals of different stature and ages: 2 pediatric ones, 4 male and 6 female adult models [-4]. It was shown that the schematic organ shapes of the MIRD-type phantoms presented an oversimplification, having an influence on the resulting dose coefficients, which in some cases deviate systematically from those calculated for voxel models [2, 5]. As a consequence of these findings, the International Commission on Radiological Protection (ICRP) decided to use voxel phantoms being the current state of the art for the update of organ dose conversion coefficients, following the current ICRP Recommendations [6]. To this end, the ICRP decided for the first time to officially adopt specific phantoms for their calculations. These phantoms should possibly accommodate all organs and tissues * Presenting author, zankl@helmholtz-muenchen.de ** Formerly: GSF National Research Center for Environment and Health

2 that have been identified as relevant source organs [7, 8] or which are known to be radiation sensitive and therefore either contribute to the quantity effective dose or are protected by separate dose limits [6, 9]. The dose conversion coefficients recommended by the ICRP are for whole populations or parts thereof, e.g. the working population, patients in medical radiation applications, or the public. Therefore, the voxel models to be adopted by ICRP should not have pronounced individual characteristics but they should as far as possible be representative of the adult Reference Male and Reference Female [] with respect to their external dimensions and their organ masses. At our working group, phantoms have been developed upon request by the ICRP that largely conform to this demand [, 2]. 2. Method to construct reference computational phantoms 2. Selection of primary image data The voxel phantoms representing the ICRP adult Reference Male and Reference Female were constructed on the basis of individual voxel phantoms segmented from whole body computed tomographic (CT) data of real patients. The principal selection criterion was that these persons should already closely resemble the external characteristics of the Reference Male and Reference Female, i.e., a body height of 76 cm and 63 cm, respectively, and a whole body mass of 73 kg and 6 kg, respectively []. This ensured that only moderate changes had to be made to the external shape of these phantoms, and thus the risk for distorting the anatomical realism could be minimised. The male phantom selected for this purpose was Golem [3] with a height of 76 cm and a weight of 69 kg. Among the previously segmented female phantoms [3] none was sufficiently close to this specification; therefore, the voxel model Laura (67 cm, 59 kg) was segmented for this specific purpose [4]. The Golem data set was stemming from a whole-body CT examination of a single average-sized 38- year old male patient and consisted of 22 slices of 256 x 256 pixels. The original voxel size was 8 mm in height with an in-plane resolution of 2.8 mm, resulting in a voxel volume of 34.6 mm individual objects were segmented (67 of these being bones or bone groups), including many but not all of the organs and tissues later identified in the ICRP characterization of the reference anatomical data []. The primary data of Laura were derived from a high resolution whole-body CT scan of a 43- year old patient of 67 cm height and a weight of 59 kg. The data set consisted of 74 slices of 5 mm width (head and trunk) and 43 slices of 2 cm width (legs), each with 256 x 256 pixels. The 2-cm slice images were re-sampled to result also in slices of 5 mm width. The resulting data set consisted of 346 slices; the voxel size was then 5 mm height with an in-plane resolution of.875 mm; this corresponds to a voxel volume of 7.6 mm organs and tissues were segmented for Laura, where 9 of them were bone regions. 2.2 Adjustment to the ICRP reference values of anatomical data The following steps were then followed: () adjustment of the body height and the skeleton mass of the segmented model to the reference data by voxel scaling, (2) adjustment of the single organ masses to the reference values by adding or subtracting a respective number of organ voxels, and (3) adjustment of the whole body mass to the reference values by adding or subtracting a respective number of adipose tissue voxels Adjustment of the skeleton First, using the reference mass data for the skeletal constituents (mineral bone, cartilage, red bone marrow, yellow bone marrow, and "miscellaneous") [] and the density data for these tissues [4], the skeleton volume aimed at was evaluated. From this value together with the number of voxels making up the segmented skeleton of the phantom, the volume of a single voxel was evaluated. The ratio of the reference body height to that of the individual voxel phantom was used as scaling factor for the voxel height; then the quotient between volume and height of a voxel was calculated, and then the in-plane resolution was evaluated as square root of this quotient. The decision to modify the skeleton only by scaling was made since we believe that the skeleton can be considered as the "frame" 2

3 that more or less defines the body shape of a person, and that thus the distortion of anatomical relations would be minimal. It was, however, not possible for both models to accommodate the entire brain mass within the skull. Therefore, it was necessary to increase the volume of the skull. Golem had a noticeably narrow head, and other organs in the head were also small compared to the reference values. Therefore, we decided to increase the voxel size of all the voxels of the entire head, and then re-sample this volume with the smaller voxel size of the rest of the body. This means that the reference phantom has a greater number of head voxels than Golem had. For the female reference phantom, only the skull was increased: The interior surface voxels of the skull were replaced by brain, and an additional layer of skull voxels was then added at the exterior surface. In order not to lose a layer of the surrounding tissues, this had to be preceded by an outward movement of the surrounding muscle, adipose tissue and skin voxels. Furthermore, an outward movement of the female phantom s ribs (as occurring also during breathing) was necessary as well to accommodate the liver. This was also done while the thickness of tissues covering the ribs muscle, adipose tissue and skin was preserved again. Apart from these unavoidable modifications of the skeleton shape, the volume of the skeleton was adjusted to the reference value by voxel scaling. Since Golem s body height corresponds to the reference value, the original voxel height was kept unmodified. Laura was taller than the ICRP adult Reference Female, so the voxel height for the female reference voxel model was reduced from 5. to 4.84 mm. After the mentioned moderate changes to the skulls of both phantoms, the numbers of segmented skeleton voxels (including the segmented cartilage) were 2427 and for the male and female reference voxel phantom, respectively. Table shows the skeleton volumes of the ICRP adult Reference Male and Reference Female as derived from the reference mass data from ICRP Publication 89 and mass density data of ICRU Report 46 [, 4]. Voxel volumes of and 5.25 mm 3 for the male and female reference voxel phantoms were evaluated from these values and the segmented skeleton voxel numbers, by dividing the volume values aimed at by the respective numbers of skeleton voxels for both phantoms. The voxel height being fixed already, this resulted in voxel in-plane resolutions of 2.37 mm for the male and.775 mm for the female phantom, respectively. Table : Reference mass values [], mass density values [4] and volumes derived for various constituents of the skeleton of the adult ICRP reference individuals. Mass (g) Mass density Volume (cm 3 ) male female (g cm -3 ) male female Mineral bone Cortical bone Trabecular bone Cartilage Red bone marrow Yellow bone marrow Miscellaneous Total Adjustment of individual organs Since all voxels of a single phantom have the same size, this voxel volume together with the number of segmented voxels for each organ and tissue resulted in a value of the volume of each organ and tissue of the scaled phantom. Multiplying these volumes with the appropriate tissue density gives the respective organ or tissue mass. In a first approach, four different types of soft tissue were considered. For these, the elemental compositions of organs with similar composition were averaged, and the densities for these tissues were averaged as well. The elemental composition and the density of Soft tissue was averaged from those of brain, heart, and kidneys; those of Soft tissue 2 from 3

4 eyes, liver, and pancreas; those of Soft tissue 3 from stomach, intestine, ovaries, spleen, testes, thyroid, and urinary bladder; and those of Soft tissue 4 from adrenals, gall bladder, oesophagus, pituitary gland, prostate, thymus, tonsils, trachea, ureters, and uterus. This procedure resulted in densities of.5 g cm -3 for Soft tissues and 2,.4 g cm -3 for Soft tissue 3, and.3 g cm -3 for Soft tissue 4 []. At a later stage, it was decided to use specific elemental compositions for most softtissue organs; but for the densities, the averaged values that were derived for soft tissues through 4 were not modified any more. The second main step of adjusting the phantoms was then to adjust the individual organ and tissue masses to the reference values by adding or subtracting a respective number of voxels. This was done by the software tool "VolumeChange" that was designed specifically for this purpose [5]. It uses the programming language IDL ("Interactive Data Language") and represents each organ by its surface voxels. The volumes are then modified by shifting surface voxels inward for decreasing, outward for increasing the respective volume. The individual organs were adjusted to the respective reference values one by one, beginning with those that were larger than reference size in order to make room for those that had to be enlarged. Some fine structures could not be adjusted exactly to the reference values, due to limitations of voxel resolution and visibility. For most organs, however, a close approximation of the reference values could be achieved. The only limitation then was due to the fact that each organ has to consist of an integer number of voxels. That means that the resulting volumes may deviate from the value aimed at by at most half a voxel volume, i.e. approximately 8.3 mm 3 for the phantom of the Reference Male and 7.6 mm 3 for that of the Reference Female. At this stage, further anatomical details were segmented in the reference computational phantoms, going back to the original CT images from which Golem and Laura had been segmented. Some effort was made to identify a larger amount of blood vessels, which was especially demanding for the male phantom, due to the relatively large slice thickness, which resulted in a decreased detectability of fine structures. Furthermore, lymphatic nodes were incorporated into the phantoms. Since these objects could not be identified on the medical images, they were drawn manually, at locations specified in anatomical textbooks [6-9]. Only a part of the lymphatic tissue reference mass was thus introduced; the distribution throughout the body and higher concentration at the specified locations was however correctly mirrored, such as in the groin, the axillae, etc. and to a certain extent also in the hollows of the knees and the crooks of the arms Adjustment of the whole body mass When the individual organs had been adjusted to their reference mass values and additional structures had been incorporated, the internal anatomy was fixed. The final step was then to adjust the whole body masses to 73 and 6 kg for the male and female reference voxel models, respectively. In both cases, the whole body masses were lower than the value aimed for, so the body had to be "wrapped" with additional layers of adipose tissue. Towards the end of this procedure, small iterations had to be made since each modification of the number of adipose tissue voxels resulted also in small changes to the skin mass, because the number of body surface voxels was modified. Finally, the whole body masses were adjusted to the reference values within. g. 3. Description of the computational phantoms of the ICRP adult Reference Male and Reference Female To clearly distinguish the reference voxel models from the models from which they originate, new names were given to them: for the male phantom, the name chosen initially by the developers was Rex (Reference adult male voxel model; Rex is also the Latin word for "king,"), and to the female phantom, the corresponding female name Regina (Latin for "queen") was given [, 2]. The ICRP did not adopt these names, and chose ICRP Adult Male (ICRP-AM) and ICRP Adult Female (ICRP-AF) instead. 4

5 3. General features of the reference computational phantoms The main characteristics of both reference adult voxel phantoms are summarized in Table 2, and graphical representations are shown in Figure (ICRP-AM) and Figure 2 (ICRP-AF). Table 2: Main characteristics of the adult Reference Male and Reference Female computational phantoms Property ICRP-AM ICRP-AF Height (cm) Weight (kg) Number of (non-zero) voxels (millions) Slice thickness (voxel height, mm) Voxel in-plane resolution (mm) Voxel volume (mm 3 ) Number of columns Number of rows Number of slices Figure : Frontal view of the ICRP-AM, the voxel phantom representing the ICRP adult Reference Male Figure 2: Frontal view of the ICRP-AF, the voxel phantom representing the ICRP adult Reference Female 3.2 Special features of the skeleton The skeleton is a highly complex structure of the body, composed of cortical bone, trabecular bone, red and yellow bone marrow, cartilage and endosteum ( bone surfaces ). The internal dimensions of most of these tissues are smaller than the resolution of a normal CT scan and, thus, these volumes cannot be segmented in the voxel models. Therefore, the skeletal dosimetry has to be based on the use of fluence-to-dose response functions that are multiplied with the particle fluence inside specific bone regions to give the dose quantities of interest to the target tissues. Nevertheless, an attempt was made to represent the gross spatial distribution of the source and target volumes in the voxel models as realistically as possible at the given voxel resolution [2]. Therefore, the skeleton was divided into those nineteen bones and bone groups for which individual data on red 5

6 bone marrow content and marrow cellularity are given in ICRP Publication 7 [2]. These individual bones were sub-segmented into an outer shell of cortical bone and the enclosed spongious part of the bone. The long bones contain a medullary cavity as third component; this is again enclosed by cortical bone. This sub-division resulted in 44 different identification numbers in the skeleton: two cortical bone and spongiosa for each of the nineteen bones mentioned above, and a medullary cavity for each of the six long bones (upper and lower half of humeri, lower arm bones, upper and lower half of femora, and lower leg bones). Furthermore, that amount of cartilage that could be identified on the CT images and could, thus, be segmented directly, was attributed to four body parts head, trunk, arms and legs. Hence, the skeleton covers a total of 48 individual identification numbers. The total volume of each bone results directly from the segmented number of voxels and the voxel volume. The cortical shell around the spongiosa was chosen to be one voxel layer; the cortical bone at the long bones shafts is thicker, and its thickness was adjusted such that the total cortical bone volume is in agreement with the reference value. For each of the nineteen bones, the spongiosa is composed of various proportions of trabecular bone, red bone marrow and yellow bone marrow. Furthermore, the additional volumes of miscellaneous [] and the not directly segmented cartilage had to be accommodated in the skeleton; for practicability, these were merged within the spongiosa volume of all skeletal sites. In ICRP Publications 7 [2] and 89 [], reference data are given for the total masses of red and yellow bone marrow, the percentage distribution of the red bone marrow among individual bones, and the bone marrow cellularity in individual bones, based on earlier data by Cristy [2]. Further data on the bone marrow distribution are not available. The volume of red bone marrow in each of the nineteen bone groups can be calculated from the reference values of the total amount of red bone marrow and its percentage distribution. The bone marrow cellularity in an individual bone gives the proportion of the entire marrow in this bone that is still haematopoietic active; that means the red bone marrow fraction. From this value, the total bone marrow volume in that bone can be calculated for all those bones with a cellularity that is non-zero. This permits then the evaluation of the volume of yellow (i.e., inactive) marrow. Accordingly, each of the nineteen bones or bone groups has its own unique bone-specific spongiosa composition. 3.3 Limitations During the process of adjustment to the reference values, some problems were encountered: as already mentioned above, the original intention to modify the skeleton exclusively by scaling could not be followed since in both cases it was not possible to accommodate the required brain mass inside the segmented skull. Therefore, the skulls of both phantoms had to be increased a bit, and for the female reference phantom, also the ribs had to be moved slightly outwards as during breathing to create enough space for the liver. Since the patients were in supine position during acquisition of the medical image data, the forces of gravity were acting differently from the situation in a standing person. Hence, the abdominal organs are shifted slightly towards the thoracic region, and hence the lungs are compressed. After consultation with the ICRP, the lung masses were adjusted to the reference values by increasing the density compared to the ICRU density value for a fully inflated lung. Further limitations of the resulting reference voxel models are due to the fact that the dimensions of some tissues are much smaller than the resolution of the available image data and could therefore () not be identified on the images (e.g., medium and small blood vessels, fine bronchial structures, and lymphatic tissue), (2) not be explicitly defined (e.g., the marrow cavities and the endosteum layer lining these cavities, the dimensions of which are only tens of micrometres), or (3) not be represented in their true size (like the mucous membranes of the extrathoracic airways). Since (a) not all small tissues could be exactly adjusted to their reference values, (b) the reference values of ICRP Publication 89 [] are rounded values, and (c) the adipose tissue was used to exactly adjust the whole body masses to the reference values, the resulting adipose tissue masses are % and 5% higher for the male and female phantom, respectively, than the corresponding reference values. 6

7 4. Selected dosimetric results The ICRP will use the computational phantoms representing the adult Reference Male and Reference Female, ICRP-AM and ICRP-AF, for a variety of dose calculations for external and internal exposures. Various workers will contribute to these results. In the following, a few selected preliminary results from our working group are presented for external photon exposures []. The exposure conditions are the following idealised geometries: broad parallel beams of mono-energetic photons that impinge on the phantoms from the front (AP), the back (PA), the left (LLAT) and the right (RLAT) side, a photon beam rotating around the phantoms length axes (ROT), as well as a fully isotropic photon irradiation (ISO). Conversion coefficeints of equivalent dose pera ir kerma free-in-air for the liver and the stomach are presented in Figure 3 and Figure 4, respectively. Conversion coefficients of the effective dose according to the recently revised definition are presented in Figure 5. Figure 3: Liver equivalent dose per air kerma free-in-air conversion coefficients for various geometries of monoenergetic photons and the reference computational models with four soft tissues, Rex (left) and Regina (right). The data are from the work of Schlattl et al. []. Liver equivalent dose / air kerma (Sv/Gy) Liver - Rex AP Rex PA Rex LLAT Rex.2 RLAT Rex ROT Rex ISO Rex.. Liver equivalent dose / air kerma (Sv/Gy) Liver - Regina AP Regina PA Regina LLAT Regina RLAT Regina.2 ROT Regina ISO Regina.. Figure 4: Stomach equivalent dose per air kerma free-in-air conversion coefficients for various geometries of monoenergetic photons and the reference computational models with four soft tissues, Rex (left) and Regina (right). The data are from the work of Schlattl et al. []..6 Stomach - Rex.8 Stomach - Regina Stomach equivalent dose / air kerma (Sv/Gy) AP Rex PA Rex LLAT Rex.2 RLAT Rex ROT Rex ISO Rex.. Stomach equivalent dose / air kerma (Sv/Gy) AP Regina PA Regina LLAT Regina RLAT Regina.2 ROT Regina ISO Regina.. 7

8 Figure 5: Effective dose per air kerma free-in-air conversion coefficients for various geometries of monoenergetic photons and the reference computational models with four soft tissues, Rex and Regina. The data are from the work of Schlattl et al. []..6 Effective dose.4 Effective dose / air kerma (Sv/Gy).2.8 AP PA LLAT RLAT.2 ROT ISO.. 5. Conclusions While in the past mathematical phantoms of the human body with simplified shapes of the body and the internal organs were used for all types of organ dose calculations, a variety of voxel models became available in recent years that are based on medical image data of real persons. It was shown by a series of studies performed by different research groups that the voxel models do not only have the advantage of a much more realistic anatomy which is quite obvious but that this difference has also a clear impact on the calculated organ doses. These findings have persuaded the ICRP to employ this new type of computational body models for the next update of dose coefficients for external and internal exposures to ionising radiation that is planned following the new ICRP Recommendations. The models "ICRP-AM" and "ICRP-AF" described in this work present the effort undertaken at our working group upon request by ICRP's DOCAL Task Group to construct voxel models representing the adult Reference Male and Reference Female. The reference voxel phantoms presented in this work are the official computational models representing the ICRP Reference Male and Reference Female. The ICRP will publish recommended values for dose coefficients for both internal and external exposures using the ICRP-AM and ICRP-AF phantoms. Acknowledgements The authors wish to express their gratitude to Dr. D. Gosch and Prof. K. Friedrich, Centre for Radiology, University of Leipzig, Germany, and to Dr. D. Hebbinghaus and Prof. B. Kimmig, Clinic for Radiation Therapy, University of Kiel, Germany, for providing the computed tomographic data. The development of the reference voxel phantoms was financially supported by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety under contract StSch REFERENCES [] PETOUSSI-HENSS, N., et al., The GSF family of voxel phantoms, Physics in Medicine and Biology 47 (22) [2] ZANKL, M., et al., Organ dose conversion coefficients for external photon irradiation of male and female voxel models, Physics in Medicine and Biology 47 4 (22)

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