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Evaluation of the Effect of a Tracheal Stent on Radiation Dose Distribution via Micro-CT Imaging

Purpose: To study the effect of a metal tracheal stent on radiation dose distribution. Method: A metal tube bracket is placed in a self-made foam tube sleeve, and micro-computed tomography scanning is performed directly. The foam sleeve containing the metal bracket is placed in a nonuniform phantom for a routine computed tomography scan. The stents in conventional computed tomography images are replaced by the stents in micro-computed tomography images. Subsequently, 2 sets of computed tomography images are obtained and then imported to a radiotherapy treatment planning system. A single photon beam at 0° is designed in a field size of 10 cm × 10 cm, a photon beam of 6 MV, and a monitor unit of 200 MU. Monte Carlo algorithm is used to calculate the dose distribution and obtain the dose curve of the central axis of the field. The dose is verified with thermoluminescence dose tablets. Results: The micro-computed tomography images of the tracheal stent are clearer and less false-like than its conventional computed tomography images. The planned dose curves of the 2 groups are similar. In comparison with the images without any stents in place, the doses at the incident surface of the stent in the conventional computed tomography images and at the stent exit surface in the rear of the stent increase by 1.86% and 2.76%, respectively. In the micro-computed tomography images, the doses at the incident surface of the stent and at the exit surface behind the stent increase by 1.32% and 1.19%, respectively. Conventional computed tomography reveals a large deviation between the measured and calculated values. Conclusion: Tracheal stent based on micro-computed tomography imaging has a less effect on radiotherapy calculation than that based on conventional computed tomography imaging.


Introduction
Tracheal bronchial stent can expand and improve a narrow airway. When patients with lung cancer and complications, such as airway obstruction, chronic asphyxia, and obstructive pneumonia, are treated with tracheobronchial stents, the symptoms of dyspnea can be immediately relieved, and ventilation function and quality of life can be significantly improved. 1,2 Accordingly, further treatment can be formulated and clinically applied.
When patients with a tracheal or bronchial stent undergo radiotherapy, metal artifacts generated by the metal component of the stent and conventional computed tomography (CT) scan cause a dose disturbance at the junction of the stent and surrounding tissues. Moreover, changes in doses at the junction can affect the tumor control rate. The effect of metal stents on dose has been studied in detail. For instance, the effect of metal stent on dose mainly depends on metallic materials, density of grids, and size of metal artifacts. 3,4 Abu Dayyeh 4 analyzed the effect of different materials on the radiation dose for esophageal cancer. The effect of nonmetallic and stainless steel stents on doses is greater than that of nickel-titanium alloy metal stents. The effect of the grid density of stents on dose is more remarkable than that of metallic materials, and the effect of a high-density grid on dose disturbance is greater than that of a low-density grid. The effect of dose deviation on the adverse reactions of the trachea needs further clinical observation and can be reduced by designing multiple radiation fields in different directions. 5,6 Evans studied the effect of a metal tracheal stent on dose and conducted an animal experiment involving the implantation of the stent to the trachea of pigs for irradiation. The measured radiation dose of the metal tracheal stent on the trachea and surrounding tissues varies from 1.8% to 6.1% within the range of 5% measurement error and without a significant difference. 7 Most studies are based on conventional CT, but conventional CT is unsuitable for microscopic stent imaging because it produces images with insufficient resolution and generates artifacts. Conversely, micro-CT has a high resolution of 1 to 100 mm, has a good microscopic function, and can obtain clear scanning images. Micro-CT adopts a tapered beam X-ray, which can obtain isotropic volumetric images and improve spatial resolution and X-ray utilization. It is widely used in tumor measurement, blood vessel extraction, and material analysis. 8,9 In our study, micro-CT is used to scan a metal scaffold and analyze the effect of tracheal stent on dose during radiotherapy. The difference between the calculated value of the planning system and the actual measured value is compared through thermoluminescence dosimetry.

Results
The metal stents scanned by conventional CT and micro-CT are shown in Figure 3. In conventional CT images, the stent structure is visible and the artifacts are heavy. In micro-CT images, the scaffold structure is clear and the artifacts are small. Figure 4 shows the dose distribution of the 2 groups. In particular, the dose curve of the 2 groups is similar, and the distribution of the dose curve around the stent varies. The dose curve of the conventional CT image group is tightened around the stent. Figure 5 shows the depth-dose curve of the center axis of the overshooting field at the central plane of the 2 groups. Dose peaks are detected at the incident face of the stent. The peak width of the dose peak around the stent in the depth-dose line planned in the conventional CT group is wider than that in the micro-CT group. The following equation is used to quantitatively compare the effects of the stent on dose and to calculate the effects of stent implantation on dose: , where PDD stent and PDD no stent represent the 100% depth dose of the mold body with and without the stent, respectively. The dose in conventional CT images increases by 1.86% at the incident surface because of the stent placement and by 2.76% at the second incident surface behind the stent compared with that of the untreated stent module. In micro-CT images, the dose increases by 1.32% at the incident surface because of the stent placement and by 1.19% at the second incident surface behind the stent. Table 1 shows the TLD data after the actual irradiation and the calculated value of the treatment planning system (TPS) at the location. The difference between the measured and calculated values is equal to (measured value À calculated value)/ measured value, and the difference between the 2 is less than 5%. Within the measurement error range of thermoluminescence dosing tablets, the difference between the calculated and measured values of the micro-CT image group is smaller than that of the conventional CT image group.

Discussion
The therapeutic planning system calculates the dose deposition of radiation in a patient's body on the basis of the relative  electron density of each body part. The relative electron density is derived from a CT image, which is transformed by the CT value-electron density curve. 11,12 The correct reading of a CT value is directly related to the accuracy of dose calculation. Metal artifacts are generated during conventional CT scanning of metal stents, and metallic stents appear thick in CT images, thereby affecting the CT value of a stent and its surrounding tissues. Micro-CT scanning can obtain isotropic volumetric images with high spatial resolution by using a cone X-ray beam. In this study, micro-CT is used to scan the metal stent, and the stent images are reduced compared with those of conventional CT scanning.
When the conventional CT scan phantom was performed, the voxel contains metal scaffolds and substitutes lung tissue, which has 2 kinds of density differences that produce a partial volume effect, because of the thickness of the scan layer. The scanning layer in a micro-CT scan is thinner than that in the conventional CT. The partial volume effect and the display rate of the stent are also greatly improved in the micro-CT scan.
The quality of CT images is related to their resolution, which is divided into spatial resolution and resolution. Spatial resolution is associated with equipment, scanning layer thickness, field of view (FOV), and image reconstruction algorithms. The thinner the scanning layer is, the smaller the voxel, the more difficult the production of partial volume effect, and the stronger the ability of an image to show small structures will be. The size of the FOV determines the image pixel size, that is, the smaller the FOV is, the more the pixels and the sharper the image will be. Conventional CT generally adopts fan-shaped X-ray beams, whereas micro-CT usually applies cone X-ray beams. A cone beam can obtain the true isotropic volume image and improve the spatial resolution. Micro-CT adopts a micro focus that is different from that of conventional CT, and its resolution can reach a micron level. It also has a good "microscopic" effect. The FOV of micro-CT is much smaller than that of conventional CT, and the resulting image quality is stronger than that of conventional CT.
When radiation enters the mold from a high-density area to a low-density area, a dose drop effect is produced because the secondary electron flux changes as the ray enters the lowdensity region from the high-density region. A dose peak is generated behind it because of the gap between the 2 materials in the mold. The density of materials simulating the lung tissue is higher than that of air. Radiation enters the relatively highdensity-simulated lung tissue from the low-density gap, forming a secondary build effect within the range of secondary electrons. The dose increases gradually as the depth increases. Conversely, the dose decreases after the effect is weakened beyond the maximum range. When incident photons pass through metal stents, an electron imbalance occurs at the incident surface. Scattering lines and secondary electrons in metal stents penetrate tissues by the backscattering of metal materials, thereby increasing the dose at the incident surface. Li 13 used an Monto Carlo (MC) model and demonstrated that the presence of esophageal metal scaffolds during esophageal cancer radiotherapy causes a 14% to 21% increase in the dose at 0.5 cm of the esophageal wall. The backscattering of metal implants is related to the material property of implants, such as the size of atomic number measured by a metal. The larger the atomic number is, the larger the scattering in the cross-section will be. 14 Abu Dayyeh 4 studied the effects of 6, 10, and 18 MV on backscattering and found no significant difference among the effects of the 3 levels on the backscattering of metal stents. Different algorithms have yielded various results on the surface dose of metal stents. The commonly used algorithms for planning systems include pencil beam (PB), collapsed cone (CC), and MC algorithm. For the dose calculation of metal implants, the CC algorithm is more accurate than the PB algorithm. 15 In this study, the MC algorithm is the most accurate for calculating the radiation dose distribution of metal implants. [16][17][18] The dose increases at the exit surface of the stent because of the scattering lines and secondary electrons generated by the original rays passing through the metal stent. The dose increases gradually as the depth increases. By contrast, the dose decreases after the effect is weakened beyond the maximum range. A peak curve is shown in the depth-dose curve at the exit surface of the stent.  In this study, the actual measured value slightly differs from the results of TPS by the MC algorithm. Within the measurement error range, this algorithm can accurately calculate the radiation dose on the surface of metal implants. The surface dose of the metal scaffold increases by 1.32%, and the dose disturbance can be further reduced through the design of multiple directional radiation fields.

Conclusion
Tracheal stent images obtained by micro-CT are clearer than those produced by conventional CT scan. The influence of tracheal stent based on a micro-CT image on the calculated result of radiotherapy is less than that based on a conventional CT image.