Xiaomin Zhang,Xu Xu,Jing Ning,Yong Yuan,Dawei Li,Yunlong Ji
Skeletal dosimetry in a microCT image-based rat model for external photon irradiation
rat model,marrow dose,heterogeneous skeletal model,skeletal dosimetry
Abstract Human skeletal dosimetry has experienced great developments in radiation protection in recent years by using the heterogeneous skeletal model. While for the rats experimentally used in radiation medicine, the investigation on skeletal dosimetry were mainly based on the homogeneous skeletal model, leading to inaccurate assessments of dose to radiosensitive tissues of red bone marrow (RBM) and bone surface. The purpose of this study is to develop a rat model with heterogeneous skeletal system and to investigate the dose difference in bone tissues for external photon irradiation. The high resolution of microCT images of a rat weighing 335 g were segmented into bone cortical, bone trabecular, bone marrow as well as other organs to construct the rat model. The absorbed dose to bone cortical, bone trabecular and bone marrow were calculated respectively by using Monte Carlo simulation for 22 external monoenergetic photon beams between 10 keV and 10 MeV under four different irradiation geometries conditions (left lateral [LL], right lateral [RL], dorsal–ventral [DV], ventral–dorsal [VD]). The calculated absorbed dose data were expressed as dose conversion coefficients and presented in this article, and the effect of irradiation conditions, photon energies and bone tissues density on the skeletal dose was discussed. The results showed that the dose conversion coefficients varying the photon energy for bone cortical, bone trabecular and bone marrow exhibit different trends and have the same sensitivity to irradiation conditions. The dose difference in bone tissues indicated that bone cortical and bone trabecular have significant attenuation effect on the energy deposition in bone marrow and bone surface for photon energies below 0.2 MeV. The set of dose conversion coefficients in this work can be used to determine the absorbed dose to skeletal system for external photon irradiation and to supplement the rat skeletal dosimetry.
Skeletal dosimetry in a microCT image-based rat model for external photon irradiation

Human skeletal dosimetry has experienced great developments in radiation protection in recent years by using the heterogeneous skeletal model. While for the rats experimentally used in radiation medicine, the investigation on skeletal dosimetry were mainly based on the homogeneous skeletal model, leading to inaccurate assessments of dose to radiosensitive tissues of red bone marrow (RBM) and bone surface. The purpose of this study is to develop a rat model with heterogeneous skeletal system and to investigate the dose difference in bone tissues for external photon irradiation. The high resolution of microCT images of a rat weighing 335 g were segmented into bone cortical, bone trabecular, bone marrow as well as other organs to construct the rat model. The absorbed dose to bone cortical, bone trabecular and bone marrow were calculated respectively by using Monte Carlo simulation for 22 external monoenergetic photon beams between 10 keV and 10 MeV under four different irradiation geometries conditions (left lateral [LL], right lateral [RL], dorsal–ventral [DV], ventral–dorsal [VD]). The calculated absorbed dose data were expressed as dose conversion coefficients and presented in this article, and the effect of irradiation conditions, photon energies and bone tissues density on the skeletal dose was discussed. The results showed that the dose conversion coefficients varying the photon energy for bone cortical, bone trabecular and bone marrow exhibit different trends and have the same sensitivity to irradiation conditions. The dose difference in bone tissues indicated that bone cortical and bone trabecular have significant attenuation effect on the energy deposition in bone marrow and bone surface for photon energies below 0.2 MeV. The set of dose conversion coefficients in this work can be used to determine the absorbed dose to skeletal system for external photon irradiation and to supplement the rat skeletal dosimetry.

Skeletal dosimetry refers to the evaluation of the absorbed dose from internal or external irradiation to two radiosensitive skeletal tissues: the red bone marrow (RBW) and bone surface. In radiation dosimetry, skeletal dosimetry is one of the most challenging works due to the rather complex and microscopic histological structure of the skeletal system, which is difficult to model during dosimetric evaluations. Over the past half-century, skeletal dosimetry, especially human skeletal dosimetry experienced great development as several research groups have contributed a lot to skeletal models and corresponding dose assessment methods. The skeletal model is a digitalized model developed to represent cortical bone, trabecular bone, bone marrow included in the skeletal system and to calculate the skeletal dose by using the Monte Carlo method. The skeletal model can be categorized as the homogeneous model and the heterogeneous model. The homogeneous skeletal model, either stylized or voxel-based, is that the whole skeletal system is defined as a homogenized tissue of fixed elemental composition and density. The heterogeneous skeletal model is a more realistic model with skeletal system segmented into the cortical bone and the spongiosa bone which either remained homogenized or further segmented into bone trabecular and bone marrow. As the homogeneous skeletal model is made of homogeneous mixture of bone cortical, bone trabecular and bone marrow, the dose assessment method with one or more scaling correction or conversion factors were introduced to adjust the calculated dosimetric results to represent the dose to radiosensitive skeletal tissues [1–3]. For the heterogeneous skeletal model, several representative methods developed by some research teams have been utilized to construct different types of heterogeneous model. The first is CTN method which utilizes the CT numbers, i.e. gray values of the skeletal pixels to create a heterogeneous skeletal model for calculating energy deposition to cortical bone and spongiosa bone [4–6]. To determine the dose to RBM and bone surface, correction factors still were used since the spongiosa bone in the CTN-based heterogeneous skeletal model is still a homogeneous tissue of trabecular bone and marrow. The second is paired-image radiation transport (PIRT) method, developed by Bolch et al. [7–9], in which two sets of tomographic images of different resolution were used to create a more anatomically realistic homogeneous skeletal model, allowing the particles in the Monte Carlo simulation to be transported through not only larger voxels of spongiosa, cortical bone and surrounding soft tissues but also tiny voxels representing the microstructure of typical spongiosa samples imaged by microCT or nuclear magnetic resonance microscopy. The PIRT method had so far represented great progress in skeletal dosimetry although it was only applied to isolated bone samples, instead of a complete skeletal embedded in a body. The third method developed by Kramer [10] was based on a fully heterogeneous and complete human skeletal model created by using macro voxels of a human skeleton and clusters of micro matrices filled with micro voxels representing segmented trabecular bone and bone marrow. The method also utilized microCT images of typical bone specimens to acquire the internal structure of spongiosa bone. Compared to human skeletal dosimetry, researchers also explored similar methods to evaluate skeletal dose for mice and rats, which are experimentally used in radiation medicine and nuclear medicine. Hui et al. [11] calculated the bone marrow dose based on a mouse model with the homogeneous tissue of skeletal. Muthuswamy et al. [12] developed a mouse bone marrow dosimetry model composed of slab, spherical and cylindrical geometries to calculate the mean energy deposited by beta particles. Stabin et al. [13] calculated the absorbed dose to bone marrow based on a homogeneous skeletal model included in a voxel mouse and a rat model. Konijnenberg et al. [14] developed a stylized rat model where the skeletal system was defined as homogeneous tissue. Bitar et al. [15] developed a mouse model containing femurs, legs marrow and other organs to calculate internal dose. Peixoto et al. [16] developed a voxel rat model including the homogeneous whole body skeleton for internal dose assessment. Zhang et al. [17, 18] developed a voxel mouse model with a homogeneous whole body skeleton to evaluate organ dose for external irradiation. Zhang et al. [19] also developed a rat model to assess organ dose for external irradiation. Since the rat model was based on a set of microCT images with resolution of 160 μm×160 μm, the trabecular in the images is invisible and the bone cortical, bone spongiosa and bone marrow were manually delineated based on the knowledge of anatomy and cannot represent the realistic anatomical structure of the bone tissues. Wu et al. [20] developed a voxel rat model with a homogeneous skeletal system to calculate organ dose for external irradiation. Xie et al. [21] extended Wu’s [20] study to further segment the skeletal system into mineral bone, RBM and yellow bone marrow (YBM) to evaluate skeletal dose for internal exposures to photons and electrons. The mouse or rat models mentioned above were mainly used for internal irradiation related to nuclear medicine, and most of their skeletal systems were defined as homogeneous tissue except the heterogeneous one in the 156 g rat model developed by Xie et al. [21]. In terms of skeletal dosimetry, especially for external irradiation, little effort was made for mice or rats compared to the human skeletal dosimetry. However, in radiation medicine which often experimentally involves the rats exposed to external radiation, it is crucial to investigate the skeletal dosimetry in detail to study dose–response relationships. Therefore, the purpose of this study is to develop a rat model containing a heterogeneous skeletal system and to conduct skeletal dosimetry research for external photon irradiation.

To develop the rat voxel model containing the heterogeneous skeletal system, a male Sprague–Dawley (SD) rat of mass 335 g was anaesthetized and scanned in a prone position by using a MicroCT named Skyscan1276 to acquire the image data. During the scanning, the scanning voltage was 85 kV and the electric current was 200 μA. To ensure the cortical bone, trabecular bone, bone marrow within each image to be visualized and identified, the scanning resolution was set as 20×20×20 μm. A total of 9200 raw slice images were obtained and each image has 4000×4000 pixels. To reduce the unnecessary voxels in the final rat model, the pixels representing the air around each image were removed automatically by a written Matlab program, and each image used for the rat model construction has 3100×1600 pixels.

The segmentation involved two steps. First, the whole skeletal system within each image was segmented by using automatic gray scale thresholding techniques. This was achieved using a written Matlab program to extract the skeletal region from the images. In addition, the rest of organs or tissues including skin, eye, brain, heart, bladder, testicle, stomach, pancreas, spleen, liver, kidneys, lungs, large intestine, small intestine, fat, muscle, etc. were segmented manually and filled with specific color with software Photoshop CS 8.0. Second, bone cortical, bone trabecular and bone marrow within images of the segmented skeletal system were further identified and segmented semi-automatically. This step involved using a low Gaussian filter to remove noise and the image binarization method to segmented mineral bone and bone marrow. Bone marrow was segmented automatically by using the written Matlab program based on the gray-scale thresholding techniques as the gray values of bone marrow are totally different from the mineral bone. After the bone marrow was segmented, the rest of the bone tissues were identified as mineral bone including bone cortical and bone trabecular. Since the gray values of the bone cortical and bone trabecular are approximate, bone cortical and bone trabecular in the mineral bone cannot be segmented automatically. As a result, bone cortical was firstly segmented manually from the mineral bone by delineating its contour as the anatomical structure of the bone cortical is comparatively simple. In the end, the rest of mineral bone was identified and segmented as bone trabecular. Figure 1 respectively represents an original transversal image of the femur with the resolution of 20×20 μm, the 3D image after the process of removing noise, binarization and 3D reconstruction, the segmented image of bone cortical, bone trabecular and bone marrow.

To conduct Monte Carlo simulation, the heterogeneous skeletal model that include the cortical bone, trabecular bone and bone marrow should be implemented in Monte Carlo code. In this study, Monte Carlo code MCNP was adopted to implement the rat model as it has a powerful repeated structure algorithm, which is very suitable for implementing models that have large number of voxels. A written Matlab program combined with the MCNP repeated structure algorithm was used to implement the whole rat model containing the heterogeneous skeletal system.

Monte Carlo code MCNPX was used to perform the skeletal dose calculation. After the construction of the rat model, the elementary composition and density of each organ or tissue of the rat model were obtained from the Adult data in the No. 46 Report of the International Commission on Radiation Units and Measurements [22]. This might introduce additional errors when the Monte Carlo simulation results were used for rat skeletal dosimetry since differences in the elementary composition and density between species of human and rat could potentially affect the energy deposition in organs or tissues. In consideration of the simulation efficiency and accuracy, the number of voxels of the rat model and the voxel size should be adjusted to a moderate level. As MCNP cannot process the huge number of voxels that reached 3100×1600×9200, the number of voxels in the rat model must be reduced to the level that the MCNP can handle. To reduce the number of voxels and also retain the anatomical structure, the only way is to increase the pixel size and reduce the number of pixels in the segmented images used for voxel rat model construction. However, the size of pixel should be reasonable so that the microstructure of the trabecular in the image can be represented as realistic as possible. To achieve this, a written Matlab program based on an image scaling algorithm was used to scale down the segmented images and increase the pixel size. In addition, a pixels-testing algorithm was developed to check and ensure that the pixels distribution of the bone cortical, bone trabecular and bone marrow in the resized image is similar to that in the original segmented image. In this work, the pixel size was increased from 20×20 μm to 200×200 μm and the number of pixels in each resized image were decreased from 3100×1600 to 310×160 correspondingly. A total of 460 resized images were used to construct the final voxel rat model, and finally the number of voxels were decreased from 3100×1600×9200 to 310×160×460. Figure 2 shows the resized image with pixel size increased from 20×20 μm to 200×200 μm. Compared to the original segmented image shown in Fig. 1(c), for example, the ratio of the number of pixels contained in bone marrow to the number of pixels contained in bone trabecular for the resized image is 67%, and is near the ratio of 68% for the original segmented image. It demonstrates that the resized image almost remain the pixels distribution of the bone cortical, bone trabecular and bone marrow in the original segmented image and the resolution of 200×200 μm is sufficient to construct the heterogeneous skeletal system. The four ideal irradiation geometric condition are respectively: left lateral (LL), right lateral (RL), dorsal–ventral (DV), ventral–dorsal (VD), as Fig. 3 shows [19]. The external monoenergetic parallel photon beam was set as 0.01, 0.015, 0.02, 0.03, 0.04, 0.05, 0.06, 0.08, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.8, 1, 2, 4, 6, 8 and 10 MeV respectively. In the simulation, a detailed physical model in MCNP was utilized to consider all the physical processes of photons and electrons. ENDF/B-VI was selected for the simulation. The tally type *F8 was selected to calculate the energy deposition in the skeletal system. The cutoff energy of photons and electrons was set as 1 keV. To keep the uncertainties of results within 5%, the simulation histories was set as 2×107 to 1×108, depending on photon energy.

The skeletal dose is calculated as the deposited energy in the bone tissue divided by its mass. The kerma free-in-air Ka for monoenergetic photon beams is calculated with equation: documented in the No. 47 report of ICRU [23], in which Ka/Φ is the ratio of kerma free-in-air to photon fluence, μtr/ρ is the conversion coefficient of mass energy and Eγ is the energy of the monoenergetic photon. The equation was used to normalize the skeletal dose to be the skeletal dose conversion coefficients, i.e. kerma free-in-air to skeletal dose.

The methods of organ segmentation usually can be classified as manual segmentation, semiautomatic segmentation and automatic segmentation. These methods will inevitably bring discrepancies between the real anatomical boundaries and the segmented boundaries, which are linked with the type and quality of images, the anatomical experience and the segmentation algorithm. In this work, all three methods have been used to segment the skeleton system and other organs or tissues. It should be noted that the uncertainties in the process of segmentation are unavoidable but are acceptable for the skeletal dose calculation. 3D visualization of the rat model and the skeletal system are shown in Figs 4 and 5. The rat model consists of a total of 22 816 000 voxels, 3 925 657 of which are external air around the rat. The size of each voxel in the rat model is 0.2×0.2×0.4 mm3. The mass and density of selected organs and tissues are shown in Table 1, in which the mass of each organ was calculated by the number of voxels times the voxel volume, and times the organ density. The total mass of the voxel rat model is about 320 g, 15 g less than the real one due to the unavoidable segmentation discrepancy of the organ or tissues as well as the organ density difference between the model and the real rat. In the heterogeneous skeletal system, the mass of bone cortical, trabecular bone and bone marrow are about 18 g, 7 g and 6 g respectively. The whole skeletal system accounts for about 9.5% of the rat model mass, which is higher than the value of 7.44% presented by Peixoto [16] and 4.44% presented by Konijnenberg [14]. While for the total marrow mass, the proportion is about 1.8%, lower than the value of 2.19% reported by Xie [21]. Since most of the skeletal system in the developed rat models belong to the homogeneous model which do not distinguish cortical bone, trabecular bone and bone marrow, the difference in skeletal mass and the marrow mass between different rat models is unavoidable. Meanwhile, individual differences in rat specimens, weights as well as ages also could lead to the proportion difference in skeletal mass. Generally, the rat model containing the heterogeneous skeletal system in this work can be used to calculate the dose to cortical bone, trabecular bone and bone marrow respectively and thus improve the rat skeletal dosimetry. Bone marrow can be categorized as YBM and RBM. In the study of Xie et al. [21], the images used for constructing the rat model are color cryo-section images, the YBM and RBM tissues in the color cryo-section images have different color information and can be distinguished by using the extracted color values. While the images used for constructing the rat model in this work are from microCT images that are reconstructed based on the density difference among tissues. Since the YBM and RBM tissues almost have the same density and the gray values of the reconstructed MicroCT images for the two types of bone marrow tissues are equal, the YBM and RBM tissues were undistinguished and considered to be uniformly distributed in the marrow cavities of trabecular bone.

Radiation biological effects are commonly classified as deterministic effects and stochastic effects. For the skeletal system, the deterministic effects are mainly osteomyelitis and hematopoietic syndrome which occurred after a certain dose level is reached. The stochastic effects occurring randomly in the skeletal system consist mainly of bone cancer and leukemia. Osteogenic cells, located on the bone surface of bone trabecular, are considered to be the target tissues for the bone cancer. Ideally, the dose to bone surface should be known to reflect the effect of radiation induced bone cancer. However, as bone surface is about 10 μm in thickness [24], it is unpractically to identify the tiny tissue in the images to perform dose calculation. Therefore, many researchers assigned the bone surface dose as the absorbed dose to homogeneous skeletal tissue although this might cause overestimation of the bone surface dose [2, 4, 25, 26]. In this work, the segmented trabecular bone was used to represent the target tissues of bone surface, and the dose to trabecular bone was regarded as bone surface dose. This method implicitly assumed that the absorbed dose to the bone surface closely adjacent to the bone trabeculae does not greatly exceed the mean absorbed dose to the bone trabecular. Although this method will result in the overestimation of the bone surface dose as the density of bone surface is lower than that of the bone trabecular, it still provides a reasonable and effective way to assess the bone surface dose. Hematopoietic stem cells, which are considered to be distributed uniformly in RBM, are sensitive to radiation and are associated with the radiation induced leukemia. Since RBM were not distinguished in this work, the average absorbed dose to bone marrow was calculated considering that YBM and RBM almost have the same density which results in the same absorbed dose to the two types of marrow. This assumption was also in agreement with ICRP’s suggestion that the average absorbed dose to bone marrow can be treated as the absorbed dose to hematopoietic stem cells. The bone trabecular dose conversion coefficients and bone marrow dose conversion coefficients are listed in Table 2–3. The relative statistic errors of all simulation results were below 5%, which shows the results were generally reliable. The values in parenthesis represent percentage of statistical uncertainties.

Figure 6–8 show the dose conversion coefficients varying with the photon energy under four irradiation conditions for bone cortical, bone trabecular and bone marrow respectively. As shown in Fig. 6, the dose conversion for bone cortical under four irradiation conditions increases with the photon energy to a peak value of about 4 ~ 6 depending on the irradiation conditions when photon energy is at 0.05 MeV, then decreases to a value of about 1.0 at 0.2 MeV and finally changes gradually in the range of from 1 to 0.4 when photon energy is higher than 0.2 MeV. This trend is mainly correlated with energy deposition varying with the photon energy in the rat bone cortical. Meanwhile, it was shown that the bone cortical dose conversion coefficients in VD and DV irradiation conditions are higher than that in LL and RL irradiation conditions especially for photon energy within 0.2 MeV due to the more tissues shielding effect in the LL and RL irradiation conditions. For bone trabecular, the Fig. 7 shows that the dose conversion coefficients fall from the peak value of about 1.6 ~ 2.2 at 0.05 MeV down to about 1.1 at 0.2 MeV, then remain stable around about 1.1 in the range of energy from 0.2 MeV to 2 MeV and continue to decrease when photon >2 MeV. It also can be seen in the Fig. 7 that the dose conversion coefficients in DV irradiation condition is highest among the four irradiation conditions within 2 MeV and then become the smallest when photon energy is above 2 MeV. This mainly results from the bone trabecular’s location, which is closer to the back of rat, and the change of photon penetrability varying the energy. In Fig. 8, the dose conversion coefficients for bone marrow change relatively slowly within 1 MeV compared to the bone cortical and bone trabecular, with peak value positioning at 1 MeV for VD, LL and RL irradiation conditions and 0.1 MeV for DV irradiation condition. However, when the photon energy >1 MeV, the dose conversion coefficients for the three bone tissues decrease gradually and have the similar trend. Figure 8 also shows that for the same reason, the bone marrow dose conversion coefficients exhibit similar sensitivity to irradiation conditions compared to that of bone trabecular.

Taking the LL and DV irradiation conditions as examples, Figs 9 and 10 show the difference in dose conversion coefficients among three bone tissues. For photon energies below 0.2 MeV, the bone tissue that has the highest dose conversion is bone cortical followed by bone trabecular, and bone marrow have the lowest dose conversion coefficients. For instance, the bone cortical dose is 2.4 times of the bone trabecular dose and 7.5 times of the bone marrow dose at 0.05 MeV in LL irradiation condition since bone cortical and bone trabecular have different levels of radiation attenuation. This indicates that the bone tissue density has a great impact on the energy deposition in the skeletal system especially for lower photon energies. While for photon energies above 0.2 MeV, the three bone tissues almost have the same energy deposition due to the higher penetrability of the photon energy, which means most photons and second electrons traversed the bone tissues and similar energy was left in the skeletal system. Therefore, compared to the homogeneous skeletal system in most developed mouse or rat model, the heterogeneous skeletal system model in the rat model of this work offered the opportunity to investigate the dose difference in different bone tissues and can provide more detailed skeletal dose information for radiation effect evaluation.

A rat model containing the heterogeneous skeletal system was developed based on the high resolution of MicroCT images to study the skeletal dosimetry. The heterogeneous skeletal in the rat model was segmented into bone cortical, bone trabecular and bone marrow, which were regarded as the dose-received tissues to investigate dose difference in bone tissues. A set of skeletal dose conversion coefficients was presented for external monoenergetic photon beams with energies from 10 keV to 10 MeV. The calculated results revealed that the dose conversion coefficients varying the photon energy for bone cortical, bone trabecular and bone marrow exhibit different trends and have the same sensitivity to irradiation conditions. The dose difference in bone tissues indicated that mineral bone (bone cortical and bone trabecular) have significant attenuation effect on the energy deposition in bone marrow and bone surface for photon energies below 0.2 MeV, which tells us that the real absorbed dose to the bone marrow and bone surface could be overestimated in the dose–effect study if the shielding effect of surrounding mineral bone was not considered carefully. The investigation on the dose conversion coefficients based on the heterogeneous skeletal system is helpful to understand the dose–effect relationship in radiation medicine. The set of dose conversion coefficients in this work can be used to determine the absorbed dose to skeletal system for external photon irradiation and to supplement the rat skeletal dosimetry.