Use of Monte Carlo Simulation Software for Calculating Effective dose in Cone Beam Computed Tomography Wilson Otto Gomes Batista Instituto Federal da Bahia Rua Emidio dos Santos s/n. Barbalho 40301-015 - Salvador, Bahia, Brasil wilsonottobatista@gmail.com Abstract This study aimed to develop a geometry of irradiation applicable to the Software PCXMC and the consequent calculation of effective dose in applications of the Computed Tomography Cone Beam (CBCT). We evaluated two different teams of CBCT for dental applications: Carestream CS 9000 3D tomograph; i-cat and GENDEX GXCB-500. Initially characterize each protocol measuring the surface kerma input and the product kerma air -area, P KA, with solid state detectors RADCAL and PTW transmission chamber. Then we introduce the technical parameters of each preset protocols and geometric conditions in the PCXMC software to obtain the values of effective dose. The calculated effective dose is within the range of 9.0 to 15.7 Sv for 3D computer 9000 CS; within the range 44.5 to 89 Sv for GXCB-500 equipment and in the range of 62-111 Sv for equipment Classical i-cat. These values were compared with results obtained dosimetry using TLD implanted in anthropomorphic phantom and are considered consistent. Os effective dose results are very sensitive to the geometry of radiation (beam position in mathematical phantom). This factor translates to a factor of fragility software usage. But it is very useful to get quick answers to regarding process optimization tool conclusions protocols. We conclude that use software PCXMC Monte Carlo simulation is useful assessment protocols for na tests CBCT in dental applications. Keywords: Monte Carlo simulation, effective dose, dental tomography. 237
1. INTRODUCTION Technology Computed Tomography Cone Beam - CBCT was introduced in the late 1990s in dental radiology. Just as in other types of radiological image there is the need to review existing protocols and / or taken to obtain images of sufficient quality for a reliable diagnosis and also with the optimization purpose for proper application of techniques that use ionizing radiation. Usually tests protocols assessments are made, from the point of view of optimization of exposures with the use of magnitude effective dose (Batista et al., 2012; INTERNATIONAL ATOMIC ENERGY AGENCY, 2004). There are several techniques to evaluate the effective dose quantity. Among these various methods include those based experimental techniques in the use of detectors type TLD and OSL inserted in anthropomorphic phantoms (Soares et al., 2015b) and also, at the present time the use of radiochromic films (Al-Okshi et al., 2013). These methods are considered extensively laborious, require extensive time, several steps up to the data acquisition and also have significant associated uncertainties (Lee et al., 2016; Soares et al., 2015a). Another option is the use of conversion factors that may be derived from the dosimetry data with the aforementioned detectors or computer simulation data and associated with direct measurements of a physical quantity that characterizes the exposure conditions, such as kerma incident air; kerma input surface; product kerma air-area etc. obtained using ionisation chambers. The values of these quantities will be used as input parameters of the computational methods (Lee et al., 2016). The software using the Monte Carlo method for calculating effective dose in radiology are an alternative for this type of evaluation for numerous radiological imaging techniques. However, for technical panoramic radiography and computed tomography cone beam in dentistry using software prepared for use in medical radiology modalities not a simple task mainly due to the geometry and rotation of the beam around the patient (Koivisto et al., 2013; Lee et al., 2016). The software PCXMC is a Monte Carlo simulation application adapted for use in personal computer developed by Radiation and Nuclear Safety Authority in Finland (Tapiovaara M, 1997). Currently this software already incorporates the new data from ICRP 103 de 2007 (ICRP, 2007) and a module for rotational techniques. In this context the objective of this work is to implement a geometry and input parameters in the software based on Monte Carlo method to assess effective dose values in computed tomography cone beam for dental applications. The results will be compared with values obtained using dosimeters (TLD) inserted in anthropomorphic phantom in previous studies and has published (Batista et al., 2015; Soares et al., 2015a, 2015b). 238
2. Materials and methods To calculate the effective dose selected five test protocols in three different equipment we have effective dose data obtained from measurements with TLDs inserted into anthropomorphic phantom. We conducted a dosimetric characterization for these protocols in terms of kerma acquired with solid state sensors RADCAL Accu Gold + with AGMS-DM+ sensor and kerma product values in the air area (PKA) measured Diamentor E2 transmission ionization chamber. 2.1 - Protocols The five protocols evaluated examinations were derived from three devices Carestream CS 9000; Gendex GXCB 500 and i-cat Classical. The protocols are identified by numbers and listed in Table 1. Table 5 Data of the five evaluated protocols. dosimetric information and geometric parameters and technical protocols. Protocol Equipment kv ma mas P KA FOV 1 FRD (mgy.cm 2 ) diameter x higher (cm) P1 P2 * P3 P4 P5 CS 9000 CS 9000 GXCB 500 i-cat Classical i-cat Classical 70 70 120 120 120 8 8 5 3-7 3-7 174 259.2 - - - 174 523 507 571 718 5.0 cm x 3.7 cm 8.0 cm x 3.7 cm 8.5 cm x 8.5 cm 16.0 cm x 8.0 cm 16.0 cm x 6.0 cm 50 50 50 52 52 1 - FOV Field of View. *This FOV is a combination of 3 FOVs with 5.0 cm x 3.7 cm The protocols with combination FOVs are common in current equipment. Thus, to evaluate a geometry has been constructed and shown below in Figure 1, as a result of the combination of three FOVs 5.0 cm x 3.7 cm. Figure 1 shows a combination of three FOVs to form an image, example of equipment Carestream CS 9000. Figure 8 Representation of the combination of three FOVs to form an image. Adapted from http://www.radiologiadigitale.com/wp-content/uploads/2014/10/fov-3ds.png 239
Zref, (cm) Zref, (cm) variable ISSSD 2016 2.2 - Calculations with PCXMC For the simulation using the software PCXMC with the supplement for rotational techniques (Koivisto et al., 2012; Tapiovaara M, 1997). The necessary input data for simulation of execution are: patient data; beam parameters; irradiation geometry. Table 2 shows the input data to perform the actual dose calculations. Table 6 Input parameters for the software PCXMC. Protocol Filtration X-ray beam Input dose kv Xref Yref Zref number (mm Al) Width (cm) Height (cm) data P1 70 3.9 5.0 3.7 0.0-2.5 P KA value P2* 70 3.9 7.6 3.7 variable P KA value P3 120 11 8.5 8.5 0.0-5.5 P KA value P4 120 11 8.5 6.0 0.0-5.5 P5 120 15 16.0 8.0 0.0-2.0 P KA value P6 120 15 16.0 6.0 0.0-2.0 P KA value * This FOV is a combination of 3 FOVs with 5.0 cm x 3.7 cm Figure 2 shows an example of geometry for generating the rotation and consequent calculation data. It is important to note that the reference point is located in the center of the FOV. That is, the focus distance reference (FRD) is the distance from the focal point to the center of the FOV. 98 96 94 92 90 88 86 84 82 80 78 76 74 72-8 -7-6 -5-4 -3-2 -1 0 1 2 3 4 5 6 7 8 Xref, (cm) Série1 FRD 98 96 94 92 90 88 86 84 82 80 78 76 74 72-8 -7-6 -5-4 -3-2 -1 0 1 2 3 4 5 6 7 8 Yref, (cm) Série Figura 10 Geometria de irradiação e representação do FOV. Figura 9 Geometria de irradiação e representação do FOV. It used a -5 degree tilt in the flow direction to represent the configuration of the beams in CBCT that limits the divergence of the beam on the lower plane, thus avoiding high exposure of the thyroid gland. 240
The calculations were performed for full 360 degree rotation in increments of 2; 5:10 degrees to determine if there are influences the number of increments. Also, has been varied-zref, ie, the Z coordinate of the center of the FOV. 3. RESULTS The results obtained using rotation in increments of 1; 2; 5:10 degree influenced the results at 2.3% in the effective dose. Thus we chose 10 degree increment used to reduce the computation time. In the Table 3 presents the effective dose values for the three protocols evaluated. Table 7 - Effective dose values for the evaluated protocols. Protocols P2 P3 P4 P5 P6 Zref Effective Dose ( Sv) 79 76.6 70.6 85.9 114.4 80 48.8 77.9 76.4 96.1 122.8 81 54.1 83.4 67.7 107.8 133.9 82 61.0 83.9 73.1 118.4 140.8 83 68.4 83.2 72.9 120.0 141.1 Individual and important contributions to effective dose assessment in dental radiology are presented in graphs, Figures 3, 4, 5, 6, 7 e 8. Figure 11 - Contributions to the effective dose of organ and tissue in Sv derived from P4 protocol. 241
Figure 12 - - Contributions to the effective dose of organ and tissue in Sv derived from P3 protocol. Figure 13 - Contributions to the effective dose of organ and tissue in Sv derived from P2 protocol. 242
Figure 14 - Contributions to the effective dose of organ and tissue in Sv derived from P5 protocol. Figure 15 - Contributions to the effective dose of organ and tissue in Sv derived from P6 protocol. The effective dose values are dependent on the position Zref. This is because depending on this position some organs or tissues may or may not be included within FOV. Figure 9 shows the dependence of the effective dose to the Zref position of the center of the FOV. 243
Effective Dose, (microgray) ISSSD 2016 160 140 120 100 80 60 40 20 0 78 79 80 81 82 83 84 Zref position, (cm) dependence with the Zref position - P4 dependence with the Zref position - P3 dependence with the Zref position - P3 dependence with the Zref position- P5 dependence with the Zref position - P6 Figura 16 - dependence of effective dose vales with the Zref position. 4. DISCUSSION The contributions of individual organs and tissues, of course, depends on or not it is included within the field of vision, ie within the radiation beam. When the image purpose is the lower jaw must be considered gland thyroid. Because this is very sensitive to radiation and thus contribute to the effective dose more significantly. After publication of the ICRP # 103 2007 with your new factors and explicitness the salivary glands and the inclusion of the oral mucosa in other tissues changed the panorama of effective dose assessments in dentistry radiology (ICRP, 2007). This dependence can be visualized through the graph, Figure 9. Comparing the effective dose values presented in Table 4 with studies conducted with TLD for these same protocols (Batista et al., 2015; Soares et al., 2015a). It is observed that the maximum and minimum differences are presented in the Table 4: 244
Table 8 - Comparing between measure and calculated the effective dose values. Maximum and minimum differences Protocols P2 P3 P4 P5 P6 Zref Effective Dose ( Sv)/ Differences (%) 79 76.6 47% 70.6 38% 85.9 37% 114.4 3% 80 48.8 10% 77.9 50% 76.4 49% 96.1 53% 122.8 10% 81 54.1 22% 83.4 60% 67.7 32% 107.8 72% 133.9 20% 82 61.0 37% 83.9 61% 73.1 43% 118.4 89% 140.8 26% 83 68.4 54% 83.2 60% 72.9 42% 120.0 91% 141.1 27% It becomes clear dependence with Zref position. It is not easy to ensure real Zref position in studies with TLDs. However, other studies show differences of the same order of magnitude between assessments with TLDs and effective dose values calculated PCXMC (Lee et al., 2016) and (Koivisto et al., 2012). Such differences appear to be significant do not invalidate the use of simulation with PCXMC software. It is a practical and useful way in demonstrating the importance of positioning and thus contributing to the optimization of protocols. 5. CONCLUSION The results confirmed the usefulness and practicality in using PCXMC software for simulation and evaluation of effective doses in dental cone beam tomography. REERENCES Al-Okshi, A., Nilsson, M., Petersson, A., Wiese, M., Lindh, C., 2013. Using gafchromic film to estimate the effective dose from dental cone beam CT and panoramic radiography. Dentomaxillofacial Radiology 42. doi:10.1259/dmfr.20120343 Batista, W.O., Soares, M.R., de Oliveira, M.V.L., Maia, A.F., Caldas, L.V.E., 2015. Assessment of protocols in cone-beam CT with symmetric and asymmetric beams usingeffective dose and air kerma-area product. Applied Radiation and Isotopes 100, 16 20. doi:10.1016/j.apradiso.2015.01.014 Batista, W.O.G., Navarro, M.V.T., Maia, A.F., 2012. Effective doses in panoramic images from conventional and CBCT equipment. Radiation Protection Dosimetry 151, 67 75. doi:10.1093/rpd/ncr454 245
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