Study of rat organ dose conversion coefficients for external photon irradiation based on voxel model and Monte Carlo simulation

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Highlights

  • Voxel rat model was constructed from Micro-CT images.

  • Rat organ dose conversion coefficients for external photon irradiation were presented.

  • The effect of size and weight difference on organ dose was investigated..

Abstract

A newly voxel rat model was developed from Micro-CT images of a rat to study rat organ dose for external photon irradiation. The voxel model with voxel size of 0.16 × 0.16 × 2 mm3 weighed about 323 g and contained most of the key organs or tissues. Monte Carlo N-particle code (MCNP) version 4C was utilized to calculate organ dose conversion coefficients for 21 external monoenergetic parallel photon beams between 10 keV and 10 MeV under four different irradiation geometries conditions (left lateral, right lateral, dorsal-ventral, ventral-dorsal). The calculated organ dose conversion coefficients were presented in tables and compared with published data based on a mouse model to investigate the effect of size and weight difference on organ dose. The calculated and comparison results show that the organ dose conversion for both mouse model and rat model exhibited similar energy dependence under the four ideal irradiation geometries, and the organ dose is sensitive to size and weight difference especially at photon energy below 0.1 MeV and above 2 MeV.

Introduction

In radiation protection and radiation dosimetry, computational phantoms that represent human anatomy were often used to simulate people exposed to external or internal irradiation. So far more than 100 human computational phantoms have been developed by different research groups to calculate radiation dose resulting from radiotherapy, occupational radiation, and accident radiation etc. However, in radiation medicine which mainly involves radiation effect study, the laboratory animals such as rats were usually exposed to radiation to study dose-effect relationships since human could not be used experimentally. To evaluate dose-effect relationship accurately, it is important to quantify organ dose for the exposed experimental animal because the radiation doses received by different organs were usually unequal. Organ dose is defined as deposited energy of radiation in the organ divided by the mass of the organ, and is not directly measurable, so organ dose conversion coefficients are utilized by linking the measurable quantities, e.g. fluence and air kerma, with organ dose. In determining the conversion coefficients, computational approaches involving the Monte Carlo simulation combined with specific experimental animal models have proved to be efficient.

In the past two decades, several mouse and rat models (mathematical model or voxel-based model) have been developed to study organ dose for internal or external irradiation. In these models, mathematical models based on simple mathematical equation just approximately describe organs and tissues. These models included the mouse model for calculating cross-organ beta doses (Hui et al., 1994), the mouse model for evaluation of radiation effect (Flynn et al., 2001), the mouse model and rat model for estimating the nuclide radiation dose to mice and rats(Funk et al., 2004), as well as the rat model for radiotherapy evaluation (Konijnenberg et al., 2004). The voxel models, which were usually developed from CT, MRI and anatomy images, describe the organs and tissues more precisely than mathematical model. These models included the real anatomical structure model from the MRI of an athymic female mouse (Kolbert et al., 2003), the voxel mouse model for absorbed fractions calculations based on CT images (Stabin et al., 2006), the 4D voxel mouse model for molecular imaging research (Segars et al., 2004), the mouse model based on the section images of a female nude mouse (Bitar et al., 2007), the four mouse phantoms used for different aspect of dosimetry (Taschereau and Chatziioannou, 2007), the rat model using CT images of an adult male Wistar rat for transport studies (Peixoto et al., 2008), the rat model for organ dose calculation (Wu et al., 2008; Xie et al., 2010), two mouse voxel phantoms with different voxel sizes for evaluating the effects of voxel size on SAFs and S-values for self-and cross-irradiation(Mohammadi and Kinase, 2011), the mouse model for organ dose calculation(Zhang et al., 2012,2016),seventeen voxel mouse models developed by MOBY software(Segars et al., 2004) with body mass ranging between 21 and 35 g for calculating absorbed fractions and S-values for positron-emitting radionuclides (Xie and Zaidi, 2013),the voxel mouse models of varying mass for studying preclinical absorbed dose response-effect relationships (Kostou et al., 2016), the voxel models of mice and rats of both sexes for internal dose assessment(Locatelli et al., 2017),the mouse model based on PET/CT images for evaluating absorbed doses during preclinical molecular imaging and targeted radionuclide therapy (Gupta et al., 2019).

In above rat models, the 139 g voxel rat model (Wu et al., 2008) had been used to calculate the organ dose conversion coefficients for external photon irradiation. However, the data is only suitable for estimating organ dose for rats with mass close to 139 g. In order to estimate organ dose for rats with other mass, more rat models with different mass need to be developed to obtain organ dose conversion coefficients. Moreover, rats with mass over 300 g are often used as exposed animals to study dose-effect relationship in radiation medicine. Therefore, this study is aimed at obtaining a set of organ dose conversion coefficients for external photon irradiation based on rat model with mass above 300 g. In the meantime, to investigate whether the organ dose for the mouse and rat show the same characteristic and how the weight and size difference affect the organ dose, comparison study on organ dose conversion coefficients based on the mouse model and rat model was also conducted.

Section snippets

Voxel rat model construction

A 335 g, male Sprague-Dawley (SD) rat was selected for the basis of constructing voxel rat model. The rat was firstly anaesthetized by intraperitoneal injection of Pelltobarbitalum Natricum, and then was scanned in a prone position by using a Micro-CT with x-ray tube setting at 80 kV and slice thickness setting at 1 mm. Thus, a total of 243 slice images were obtained in the form of Bitmap (BMP); after removed the useless pixelsrepresentingair around each image, the final images used for model

Voxel rat model

Except the 1,345,844 voxels that represent external air around the rat, the voxel rat model was comprised of a total of 6,173,302 voxels with size of 0.16 × 0.16 × 2 mm3, and contained most of the main organs or tissues of the rat. 3D visualization of the some internal organs of the voxel rat model are shown in Fig. 3. The mass of each organ equals to the number of voxels times the voxel volume, and times the organ density. Table 1 shows the mass and density of selected organs and tissues. The

Conclusion

A set of organ dose conversion coefficients based on a newly developed rat model with mass over 300 g was presented for external monoenergetic parallel photon beams with energy from 10 keV to 10 MeV. The calculated results revealed that the organ dose conversion coefficients varying the photon energy show similar trend for most internal organs. For those organs and tissues which have asymmetric locations and orientation, organ dose exhibits varying degrees of sensitivity to irradiation

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the Chinese National Science Foundation Project (11305267).

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