Evaluation of the angular dependence of the nanoDot OSL dosimeter toward direct measurement of the entrance skin dose Poster No.:
C-0721
Congress:
ECR 2015
Type:
Scientific Exhibit
Authors:
T. Okazaki , H. Hayashi , K. Takegami , H. Okino , K.
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2
2 1
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Nakagawa ; Tsukuba, Ibaraki/JP, Tokushima/JP Keywords:
Dosimetric comparison, Dosimetry, Conventional radiography, Radioprotection / Radiation dose
DOI:
10.1594/ecr2015/C-0721
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Aims and objectives In an X-ray diagnostics, evaluating an entrance skin dose, schematically represented in Fig. 1, is important because the dose of exposure becomes highest in that position. The entrance skin dose, however, is not measured in typical clinical conditions because typically used dosimeters might adversely affect medical images. That is, the image of the dosimeter is superimposed on the diagnostic X-ray image. The air-kerma is, therefore, measured with an ionization chamber, and then a correction on the back scattering [1-3] is performed to obtain the actual entrance skin dose. This method is adopted as the general method as shown in Fig. 2. A small-type OSL (Optically Stimulated Luminescence) dosimeter, nanoDot, is commercialized by Landauer Inc. Its main application is recording the absorbed doses during radiation therapy procedure usually at high energies of X-rays. [4-6]. Neverthless, using nanoDot in diagnostic X-ray imaging has not been extensively tested and only a small number of papers were reported [7-9]. In this paper we study the application of nanoDot to measure the entrance skin dose because the dosimeter has low X-ray absorption and is not as visible on the images. Fig. 3 on page 4 exemplifies a phantom study using the nanoDot dosimeter. As shown in the lower picture, the nanoDot put on an elbow cannot be seen in a radiogra#c image. At the same time it shows the ability of nanoDot to directly measure an entrance skin dose [10] (see Fig. 2). In order to introduce a dosimeter into clinical application, knowledge of the basic properties such as angular and energy dependences are essential. We focus our attention on the angular dependence. In entrance skin dose measurement of diagnostic X-rays, the angle of incidence of the X-rays to the dosimeter varies depending on the side of the X-ray tube and the wearing point of the dosimeter. Fig. 4 on page 5represents some examples. In chest radiography, it is desirable to put the dosimeter in the center of the irradiated field (X-ray is perpendicular to the dosimeter in Fig. 4 on page 5a). The wearing position, however, depends on the circumstances and the dosimeter can be attached to a side of the human body in some cases (Fig. 4 on page 5b). The dosimeter can also be irradiated in 4# geometry as in an application of CT imaging ( Fig. 4 on page 5c). The aim of the present study is to evaluate the angular dependence of the nanoDot OSL dosimeter.
Images for this section:
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Fig. 1: Schematic drawing of the exposure dose. We focused our attention on the entrance skin dose.
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Fig. 2: Schematic diagram of dose measurement. Our method can measure the dose of exposure of both incident X-ray and back-scattered X-ray.
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Fig. 3: Characteristics of nanoDot Optically Stimulated Luminescence (OSL) dosimeter. As shown in the lower photograph and radiography image, the nanoDot dosimeter did not affect a medical image.
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Fig. 4: Importance of angular dependences data. In case the dosimeter is set at the center of irradiation field of chest radiography, angular dependences data is not necessary. In a X-ray CT, a dosimeter is exposed by X-rays from all directions.
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Methods and materials First of all, we developed an annealing device to zero OSL dosimeters before their use (Fig. 5 on page 8)[11]. Our annealing device consists of four fluorescent light tubes emitting light in blue region and can anneal eighty dosimeters simultaneously. It enables the use of sufficiently annealed dosimeters for the following experiment. Secondly, we designed and constructed an experimental setup based on commonly used diagnostic X-ray apparatus (Toshiba medical systems corporation, MRAD-A 50S/70). Fig. 6 on page 8shows photographs of the experimental setup. In order to reduce scattered X-rays, the original collimator, made by Takegami et al. [12], was installed in front of the movable diaphragm. This collimator is composed of four lead-shields (100 mm wide, 80 mm high, and 2 mm thick), and each one is supported by a 2 mm thick aluminum plate. The collimator works not only to narrow the X-ray beam but also to reduce the number of scattered X-rays from the movable diaphragm. In the present study we set the collimator aperture to 5.0 mm so as to obtain a beam of 20 mm in diameter at the source-to-detector distance of 150 cm. The following condition was used to irradiate the dosimeters: Vp = 40-140 kV and tube current-time product equal to 100 mA.s. The dosimeter was fixed to the rotational unit. The main element of this unit was an industrial robot (IAI corporation, RCP2-RTCSL), which can be remotely controlled by a personal computer. The accuracy of the rotation control is approximately 0.05° which is sufficient for our experiment. On the industrial robot, we set an original attachment made of aluminum frame components. As shown in Fig. 6 on page 8b, a thin film (Pechiney Plastic Packing Company, ParafilmTM) is fixed in the air using nylon lines tightened between the two arms. The thickness of this film, several tens of micrometers, is so small that it does not affect the X-ray absorption or scattering. The dosimeter is attached at the center of the film. In order to prevent generation of scattered X-rays from the aluminum attachment, it was used in two different settings: in setup 1 the film is set parallel to the aluminum frame as represented by Fig. 6 on page 8b-1, and in setup 2 the film is set perpendicular to the frame as represented by Fig. 6 on page 8b-2. The angular dependences of 0º ± 45º and 180º ± 45º were measured by setup 1, and those of 90º ± 45º and 270º ± 45º were measured by setup 2. Data was recorded for every 15º. Fig. 7 on page 9ashows a front view of the nanoDot OSL dosimeter, and Fig. 7 on page 9b represents the definition of the rotational angles; we define two angles as theta (#) and phi (#). Fig. 7 on page 9cshows simplified constructions of the nanoDot OSL dosimeter which is used in the Monte-Carlo simulation. Thirdly, we simulate the angular dependences of the dosimeter by Monte-Carlo simulation code EGS5 [13] which is used for evaluating low-energy photon/electron transport. Okino reported that the code agreeably reproduced the experiments in the diagnostic X-ray region of energies [14]. Fig. 8 on page 10 shows a simulated condition. The necessary information of size, density, and composition was published by Page 7 of 22
Lehmann et al. [6]. We used these data as a reference. In the simulation, X-ray spectra [15-17] of diagnostic X-ray apparatus were reproduced by the Monte-Carlo method as shown in Fig. 9. Lastly, we compared the experimental data and the simulated ones. For the comparison, each data were normalized by the result of 0º in # and # directions.
Images for this section:
Fig. 5: The Annealing device was developed by Nakagawa et al. It enables to use sufficiently annealed dosimeters for our experiments.
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Fig. 6: Experimental setup equipment. Our experimental technique uses (a) collimator and (b) rotational unit.
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Fig. 7: Definition of the angles in the present study.
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Fig. 8: The angular dependences are calculated by the EGS5 Monte-Carlo simulation code. To simulate the angular dependences accurately, refer the dimension of the nanoDot dosimeter which was precisely measured by a micro-CT.
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Fig. 9: X-ray spectra of 40-140 kV. These spectra were calculated by Birch's formula.
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Results Fig. 10 and Fig. 11 on page 14 show the comparison of experimental and simulated angular dependences for # and # directions, respectively. Solid circles show experimental data, and each of them has approximately 5% error bar which includes 5% of a systematical uncertainty [9] and less than 1% of a statistical uncertainty. Dashed lines show simulated data of which statistical uncertainty is negligibly small. The experimental data were in good agreement with the simulated ones except for data at # = 270º. Fig. 12 on page 15 shows comparison between our results and the previously published data by Al-Senan et al [8]. At 80 kVp their data seems to be in agreement with our data, but we find disagreement at 120 kVp. Although the referred published data has much smaller uncertainty compared with our data, the error bar of their data can be underestimated. One reason of the disagreement is considered to be scattered X-rays. In our study, an incident X-ray beam is collimated well and scattered X-rays are reduced by means of a custom-made collimator [12]. In contrast, there is no description about a reduction of the scattered X-ray in their published paper [8]. We, therefore, presume they irradiated the dosimeter without reducing the scattering of X-rays. Here, we shall discuss the necessity of the collimator in detail. Fig. 13 shows a comparison of results in two different experimental conditions. Solid circles and a dashed line are the same data represented in Fig. 10. The open circles show a result of the experiment performed without the collimator. The difference is clearly shown in the plot; the open circles are not in good agreement with the present experimental and simulated data especially at 90º and 270º. From these facts, we concluded that a collimator to reduce scattered X-ray from the diaphragm of X-ray apparatus [18] should be installed. Our results indicate that the nanoDot OSL dosimeter can be a powerful tool to measure the entrance skin dose because this detector has a good (almost flat) response within the all angles between the dosimeter and an incident X-ray axis. In the direction of # = 90º and 270º and # = 90º and 270º, the detection efficiencies reduce to about 80% compared with 0º. We should pay attention to this phenomenon for the clinical application because the dosimeter can be used as shown in Fig. 4 on page 17b. In such use of the dosimeter, further research about the reduction in detection efficiency is needed. Finally, we present a simple simulation of the dosimeter randomly irradiated within 4# of angles of incidence by the EGS5 code. Fig. 14 shows the results of normalized detection efficiency ratio of randomly irradiation data divided by the data at # = 0º and # = 0º. We can find that the differences, at most 4%, are negligibly small. We, therefore,
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conclude that the nanoDot OSL dosimeter does not need to take into account the angular dependences for the general use in a clinical application. Images for this section:
Fig. 10: Comparison between experimental and simulated data in # direction of rotation.
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Fig. 11: Comparison between experimental and simulated data in # direction.
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Fig. 12: Comparison between our data and Al-Senan's data. In 80 kVp, their data seem to be in agreement with ours. In contrast, large differences are found in 120 kVp.
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Fig. 13: Necessity of a collimator. In order to reduce scattered X-ray generated by the movable diaphragm of X-ray apparatus, installed a custom-made collimator.
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Fig. 4: Importance of angular dependences data. In case the dosimeter is set at the center of irradiation field of chest radiography, angular dependences data is not necessary. In a X-ray CT, a dosimeter is exposed by X-rays from all directions.
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Fig. 14: Comparison between a data at # = 0º and that of dosimeter randomly irradiated within 4-# directions.
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Conclusion We constructed a novel experimental setup of equipment to evaluate the angular dependence of the nanoDot OSL dosimeter. Irradiation using a narrow X-ray beam with low scattered radiation enables the evaluation of angular dependence more precisely and also more accurately. We believe that our results and data are valuable in clinical application for direct measurement of entrance skin dose using nanoDot OSL dosimeter.
Acknowledgement The authors would like to thank Mr. H. Sekiguchi for supporting this work. Also, authors would like to thank Ms. N. Kimoto and Ms. I. Maehata for their help in writing the manuscript.
Personal information
Okazaki Tohru Nagase Landauer, LTD., Technical Office, Tsukuba, Ibaraki;
[email protected] Hayashi Hiroaki Institute of Health Biosciences, Tokushima University, Tokushima, Tokushima;
[email protected] Takegami Kazuki Graduate School of Health Sciences, Tokushim University, Tokushima, Tokushima Okino Hiroki School of Health Sciences, Tokushim University, Tokushima, Tokushima
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Nakagawa Kohei School of Health Sciences, Tokushim University, Tokushima, Tokushima
References
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