Construction of Voxel-type Phantom Based on Computed Tomographic Data of RANDO Phantom for the Monte Carlo Simulations

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1 Construction of Voxel-type Phantom Based on Computed Tomographic Data of RANDO Phantom for the Monte Carlo Simulations K. Minami 1, K. Ejiri 1, M. Shimo 1, M. Kato, Y. Takeuchi, K. Yonemochi, H. Toyama and K. Katada 1 Faculty of Radiological Technology, Fujita Health University School of Health Sciences, , Toyoake, Japan kminami@fujita-hu.ac.jp Fujita Health University Hospital, , Toyoake, Japan Department of Radiology, Fujita Health University School of Medicine, , Toyoake, Japan Abstract. The Monte Carlo simulations using the mathematical phantom of a human body model is useful as a method of getting to know the details of the radiation exposure in a human body. However, it is very difficult to evaluate experimentally the validity of the calculation result of the simulation using the human body model. Then, the mathematical phantom of voxel-type (voxel phantom) for the Monte Carlo simulations was constructed from Computed Tomographic (CT) data of this phantom using a human body phantom which can actually be measured. The Alderson RANDO phantom RAN-1 type (female model) made by The Phantom Laboratories, Inc. was selected as the target of this study. Electron Gamma Shower version 4 (EGS4) which is the Monte Carlo transport code was used for the simulation of the radiation dose distribution. Some kinds of voxel phantoms with different voxel size were created, and the radiation dose distribution obtained in the simulation was considered about capability expressed finely. Moreover, 4MV X-ray is directly irradiated from the outside at the RANDO phantom, the simulation result in the same conditions as an actual measurement is compared. The fluorescence glass dosimeter (Dose Ace made by Asahi Technoglass corp.) was used for dose evaluation by the measurement. The dose distribution of the voxel phantom by EGS4 simulation expressed more detailed dose distribution structure as voxel size became fine. In comparison of the measured value and the calculated value, it differed 1 % at the maximum. The meaning which builds a voxel phantom from a RANDO phantom with the structure more near a human body is very effective in order to bury the gap of an actual measurement and a simulation, and in order to evaluate the accuracy of the human body model building method, it offers many information. 1. Introduction Since it is not easy to investigate the details of the radiation exposure in a human body by measurement, the Monte Carlo simulations method has been applied about human body models various until now [1, ]. Although this method is useful as a method instead of actual measurement, the calculation accuracy of a dose is required in order that evaluating construction accuracy and improving, since it is dependent on the reproducibility of the built human body model may raise dose accuracy. Then, the mathematical phantom of voxel-type (voxel phantom) for the Monte Carlo simulations was constructed from Computed Tomographic (CT) data of this phantom using a body phantom which consists of natural human skeleton and tissue-equivalent material as a model which can perform this accuracy evaluation. The calculation value by the simulation which used built voxel phantom, and the actual measurement using the human body phantom were compared by the same irradiation conditions, and the construction accuracy was evaluated.. Materials and Methods.1 Human body phantom The Alderson RANDO (Radiation Analog Dosimetry) phantom RAN-1 type made by The Phantom Laboratories, Inc. was selected as the target of this study (Fig.1). This phantom is the female model of 16 cm height and 54 kg weight, and consists of parts of 5 slices. The thickness of each slice is Slice Present address: 1-98 Dengakugakubo, Kutsukake-cho, Toyoake, Aichi, (Japan). 1

2 No.1: 4 cm/slice, Slice No.-4:.5 cm/slice, Slice No.5: 1 cm/slice. The main composition elements of the phantom are five substances of Air or Cavity, Lung, Soft tissue, Bone, and Acrylic. The inside of this phantom has many holes (Hole Grid: 868 holes) which can insert the detector for measuring a depths dose, and the Mix DP plug is usually inserted.. Construction of voxel phantom First, the RANDO phantom was scanned with X-ray CT equipment Aquilion made by Toshiba Medical Systems Corp., and CT image data (DICOM data: pixels, 5 mm/slice, slices) was acquired. Next, the area of each slice was set up on conditions as shown in Fig. from CT value of obtained CT image data. Moreover, since the area of Bone and Soft tissue had a large change of CT value, Bone was divided into areas of 16 steps, and Soft tissue was divided into 5 steps. (For example, change of CT value of Bone was large between the inside of Bone and the Bone surface, and Soft tissue was large near the surface of the RANDO phantom.) Since the value of the detector inserted in Hole Grid reflected FIG. 1. Alderson RANDO Phantom (RAN-1 TYPE). the dose of the substance around the Hole Grid, the area of Hole Gird was transposed to the composition element of the RANDO phantom, judging from the substance around the Hole Grid. Voxel phantoms of 4 types were created and the voxel size was taken as mm, mm, mm, and mm, respectively (Fig.). The original program created by Microsoft Visual Basic 6. (SP5) performed processing of the setup of these areas. Analysis of data was performed by the Scion Image version 4.. Beta. Lung Air Cavity S1 S S S4 S5 Acrylic Soft tissue: S1 S5 Bone: B1 B16 B 1 B B B 4 B 5 B 6 B 7 B 8 B 9 B1 B11 B1 B1 B14 B15 B FIG.. The area setting of the RANDO Phantom by CT value. 14 CT Value. Monte Carlo simulations used voxel phantom Electron Gamma Shower version 4 (EGS4) [] was used for the Monte Carlo transport code for dose evaluation. The constructed voxel phantom carried out the simulation by including in EGS4. Elemental composition and density of five kinds of substances (Air, Lung: Alderson lung, Soft tissue: Alderson muscle, Bone: B1, Acrylic) used in EGS4 simulation were created based on ICRU Report 44 [4]. However, the density of Bone (16 areas) and Soft tissue (5 areas) was calculated in quest of the relation between CT value and density from the data of the CT Test Phantom CT-B type made by Kyotokagaku Corp. beforehand scanned with the same CT equipment. Personal Computer (PC) used for the simulation is Central Processing Unit (CPU): 1 GHz (Pentium III), Memory: 51 MB and Operating System (OS): Linux 8. made by Red Hat, Inc..4 Examination of voxel size used voxel phantom EGS4 simulation was carried out on the same conditions using four kinds of voxel phantoms from which voxel size differed. The simulation included the voxel phantom of the breast of only one slice shown in ()-(5) of Fig. in EGS4, respectively. Each number of calculation cases was set to histories 5 batches, and irradiated 4MV X-ray equally in the direction of the back from the front. Fig.4 is 4MV X-ray spectrum used in the simulation, and the spectrum was created based on Medical

3 Linear Accelerator of MEVATRON KD /5 PRIMUS made by Siemens Ltd. The 4MV X-ray spectrum was calculated from the approximation formula of Schiff [5] by having set target material to Gold, and was calculated as a spectrum after being filtered with the flattening filter (Stainless Steel) of average thickness 7.7 mm []. The radiation dose distribution was created from the result obtained by the simulation, and the power of expression was examined. R L Acrylic (1) CT image Voxel size:.9.9 5mm Number of pixel: () Voxel Phantom () Voxel Phantom Voxel size:.9.9 5mm Voxel size: mm Number of pixel: Number of pixel: [CT Value] 14 Column (4) Voxel Phantom (5) Voxel Phantom Voxel size:.6.6 5mm Voxel size: mm Number of pixel: Number of pixel : FIG.. CT image and voxel phantom image of the breast. Row [ Direction of pixel ].5 Accuracy evaluation of voxel phantom In order to evaluate the construction accuracy of built voxel phantom, the measured value using RANDO phantom was compared with the calculated value acquired by the EGS4 simulation. Actual measurement inserted the fluorescence glass dosimeter GD-5 (Dose Ace made by Asahi Technoglass Corp.) with Sn-filter in the inside of the RANDO phantom, and the depths dose was measured by irradiating 4MV X-ray of Medical Linear Accelerator (MEVATRON KD /5 Relative Photon Fluence PRIMUS) at the whole body of the phantom. The irradiated dose was set to 1 Gy (absorbed dose of air at cm), and the measurement was carried out times. When the whole body of voxel phantom was targeted, the simulation included the phantom of voxel size: mm (18 18 pixels) in EGS4 from the relation of the memory capacity of PC, and irradiated 4MV X-ray shown in Fig.4. The number of calculation cases in one simulation was set to histories 5 batches, and the simulation was carried out times by changing random numbers. The geometry for the irradiation in actual measurement and EGS4 simulation set distance from a source to the centre of a phantom to cm, as shown in Fig.5, and it carried out whole body irradiation in the direction of the back from the Energy [ MeV ] FIG. 4. 4MV X-ray spectrum.

4 front of a phantom. Absorbed dose (Gy) obtained in actual measurement and EGS4 simulation, all the doses were standardized to absorbed dose of the datum point in the phantom shown in Fig.5. Medical Linear Accelerator cm Datum point 4MV X-ray Bed FIG. 5. Irradiation geometry. Datum point : First cervical vertebrae level Irradiation direction Anterior Posterior (AP) Measurement Field size : cm Calculation Field size : 1 6 cm. Results.1 Comparison of voxel size Fig.6 shows the radiation dose distribution and line profile curves of the breast of four kinds of voxel phantoms from which voxel size differed. The dose distribution of voxel phantom by EGS4 simulation expressed more detailed dose distribution structure as voxel size became fine. Especially the line profile curve of acrylic part was expressed in detail by voxel size of mm (56 56 pixels) or less. The form of another line profile curve other than the acrylic part was comparatively well alike even if voxel size differed, as shown in Fig.6.. Comparison of measurement value and calculation value Fig.7 is an example which showed the radiation dose distribution when irradiating 4MV X-ray in EGS4 simulation at the whole body of voxel phantom. Fig.8 shows the line profile curve of dose distribution using voxel phantom, and the measured value using RANDO phantom in LEVEL I-IV shown by Fig.7. The details of the measured value of a measurement point and the calculated value shown in Fig.8 are shown in Table 1. In LEVEL I, the calculated values over the measured value differed % at the maximum. In LEVEL II, it differed % at the maximum. In LEVEL III, it differed 1 % at the maximum. Moreover, near the irradiation field centre of the LEVEL II circumference, the difference in a calculated value to a measured value was great compared with other measurement points. In LEVEL IV, it differed 8 % at the maximum. 4. Discussion Development of a voxel phantom is very useful as a means to make mathematical phantom precise [1, ]. However, there is almost nothing that checked the validity of a calculation result by experiment in the simulations using the voxel phantom performed now. Since a RANDO phantom which can actually measure a depths dose is an object, voxel phantom built this time can perform evaluation of the construction accuracy or the construction technique. The more voxel size of voxel phantom used in a simulation was small, since there was little influence of the partial volume effect, the more the power of expression (resolution) improved. However, when the voxel size is small, the number of incidence photons per voxel decreases, and there is a demerit that signal to noise ratio (S/N) falls. In order to raise the S/N, the number of incidence photons must be increased, and huge calculation time is needed. Moreover, in order that reduction of voxel size may cause the increase in the number of voxel, as a result, the memory capacity of a vast quantity of PC is needed. Therefore, it is necessary to determine voxel size used in the simulation of the radiation dose distribution in consideration of both size of the region made into the purpose, and memory capacity of PC used for calculation. 4

5 Line Profile Line Profile Line Profile Line Profile 1 (1) pixels, 5mm/slice ( voxel size : mm ) 1 () pixels, 5mm/slice ( voxel size : mm ) 1 () pixels, 5mm/slice ( voxel size : mm ) 1 (4) pixels, 5mm/slice ( voxel size : mm ) FIG. 6. Dose distribution and line profile curve of the breast. 5

6 LEVEL I 1 LEVEL II LEVEL III LEVEL IV LEVEL I 1 LEVEL I : Brain level LEVEL II : Sixth thoracic vertebrae level LEVEL III : Xiphoid process level LEVEL IV : Forth lumbar vertebrae level 1 LEVEL II 1 LEVEL III 1 Sagittal Coronal LEVEL IV FIG. 7. Dose distribution of voxel phantom irradiated by 4MV X-ray. 6

7 1 4 Relative value calculation measurement : Measured Line Profile point LEVEL I Line Profile 1 LEVEL II Line Profile LEVEL III Line Profile 1 4 LEVEL IV Relative value Relative value Relative value. 4 6 Pixel No. of Row direction Pixel No. of Row direction Pixel No. of Row direction calculation measurement calculation measurement calculation measurement. 4 6 Pixel No. of Row direction FIG. 8. Measured value by glass dosimeter and calcurated value by EGS4. 7

8 Table 1. Comparison of measured value and calculated value. LEVEL Measured Measurement (A) Calculation (B) Point No. Mean ± S.D. CV(%) Mean ± S.D. CV(%) (B) (A) ± ± I.964 ± ± ± ± ± ± ± ± II 1.8 ± ± ± ± ± ± ± ± III.956 ± ± ± ± ± ± ± ± IV 1.1 ± ± ± ± ± ± Measured value and calculated value is the relative value standardized in the datum point. As shown in Fig.8 and Table 1, it is thought that the cause which the disagreement more than 1 % produced in comparison of a measured value and a calculated value near the irradiation field centre of the LEVEL II circumference has a problem in 4MV X-ray spectrum used in the simulation. Because, as shown in Fig.9, the value of Percentage Depth Dose (PDD) of EGS4 simulation using the 4MV X-ray spectrum used this time had the difference in 1 % grade at the maximum to the value of actually measured PDD. It is thought that this cause is because the special conic flattening filter used in the generating process of X-ray was used as average thickness 7.7 mm. In actual measurement, it is thought on the form of the flattening filter that the radiation quality near an irradiation field centre is made hard compared with other irradiation fields. Therefore, before this study estimated the construction accuracy of built voxel phantom, it was suggested that a problem is in the accuracy of irradiation conditions of a simulation. 5. Conclusion PDD [ % ] 1 5 Calculation by EGS4 Measurement by Reference disimeter Depth [ cm ] FIG. 9. Percentage Depth Dose Curve. The meaning which builds a voxel phantom from a RANDO phantom with the structure more near a human body is very effective to bury the gap of an actual measurement and a simulation, and in order to evaluate the accuracy of the human body model building method, it offers many information. However, in order to estimate the construction accuracy of built voxel phantom for this study, it was suggested that it is necessary to raise another accuracy other than the construction accuracy, such as irradiation conditions. Therefore, it is necessary to verify these conditions by the simpler system and to apply to the voxel phantom after that. 6. References 8

9 1. K. Saito, A. Wittmann, S. Koga, Y. Ida, T. Kamei, J. Funabiki and M. Zankl, Construction of a computed tomographic phantom for a Japanese male adult and dose calculation system. Radiat Environ Biophys, Vol.4, 69-76, (1).. T.C. Chao, A. Bozkurt and X.G. Xu, Conversion coefficients based on the VIP-MAN anatomical model and EGS4-VLSI code for external monoenergetic photons from 1 kev to 1 MeV. Health Physics, Vol.81(), 16-18, (1).. W.R. Nelson, H. Hirayama and D.H.O. Rogers, The EGS4 code System, SLAC-65, Stanford Linear Accelerator Center, Stanford, Calif (1985). 4. International Commission on Radiation Units and Measurements, Tissue Substitutes in Radiation Dosimetry and Measurement. Report 44. Maryland (1996). 5. Schiff LI., Energy-angle distribution of thin target bremsstrahlung. Phys.Rev., Vol.8(), 5-5, (1951). 6. H. Kato, The Monte Carlo simulation with a personal computer. Japanese Journal of Radiological Technology, Vol.55(), , (1999). 9