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Dosimetric shield evaluation with tungsten sheet in 4, 6, and 9 MeV electron beams
Physica Medica 30 (2014) 838e842
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Physica Medica journal homepage: http://www.physicamedica.com
Technical notes
Dosimetric shield evaluation with tungsten sheet in 4, 6, and 9 MeV electron beams Takahiro Fujimoto a, Hajime Monzen b, *, Manabu Nakata a, Takashi Okada a, Shinsuke Yano a, Toru Takakura a, Junichi Kuwahara a, Makoto Sasaki a, Kyoji Higashimura a, Masahiro Hiraoka b a
Clinical Radiology Service Division, Kyoto University Hospital, Kyoto 606-8397, Japan Department of Radiation Oncology and Image-Applied Therapy, Kyoto University Graduate School of Medicine, 54 Kawara-cho, Shogoin, Sakyo-ku, Kyoto 606-8507, Japan
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 February 2014 Received in revised form 25 May 2014 Accepted 27 May 2014 Available online 19 June 2014
In electron radiotherapy, shielding material is required to attenuate beam and scatter. A newly introduced shielding material, tungsten functional paper (TFP), has been anticipated to become a very useful device that is lead-free, light, flexible, and easily processed, containing very fine tungsten powder at as much as 80% by weight. The purpose of this study was to investigate the dosimetric changes due to TFP shielding for electron beams. TFP (thickness 0e15 mm) was placed on water or a water-equivalent phantom. Percentage depth ionization and transmission were measured for 4, 6, and 9 MeV electron beams. Off-center ratio was also measured using film dosimetry at depth of dose maximum under similar conditions. Then, beam profiles and transmission with two shielding materials, TFP and lead, were evaluated. Reductions of 95% by using TFP at 0.5 cm depth occurred at 4, 9, and 15 mm with 4, 6, and 9 MeV electron beams, respectively. It is found that the dose tend to increase at the field edge shaped with TFP, which might be influenced by the thickness. TFP has several unique features and is very promising as a useful tool for radiation protection for electron beams, among others. © 2014 Associazione Italiana di Fisica Medica. Published by Elsevier Ltd. All rights reserved.
Keywords: Tungsten paper Electron beam Transmission Radiation shield
Introduction In electron radiotherapy, shields are placed above the patient's body surface to block beams and scattered rays. By appropriately shaping the shield, which is most commonly made of lead or lowmelting point alloy (LMA), radiation can be concentrated to the appropriate area by forming an irradiation field. There have been a number of reports on the dose characteristics of electron beams with lead shields, and the dose is considered to depend on the energy, size of the irradiation field, and thickness of the shield material [1e7]. Lead has excellent shielding properties against electron beams and is available at a reasonable price. However, its effects on the environment and toxicity to the human body have been identified as problems [8]. While there have been reports on the use of elements with a high atomic number, such as tungsten, as substitutes for lead [9e13], they are not commonly used owing to the difficulty in processing and their high cost. Recently, rubber or
* Corresponding author. Tel.: þ81 751 3762; fax: þ81 771 9749. E-mail address: [email protected] (H. Monzen).
plastic containing tungsten powder, which has advantages including easy processability, has been developed. In this study, the shielding performance of tungsten functional paper (TFP) newly developed as a shielding material against electron beams was evaluated. The energy of electron beams and changes in their dose characteristics with the size of the irradiation field were also measured to evaluate the possibility of its clinical use. Materials and methods TFP prepared by Toppan Printing Co., Ltd., is a sheet-form shielding material 0.3 mm in thickness containing tungsten powder at about 80% by weight. The element ratios of TFP (mol%) are H: 24.2%, C: 40.4%, O: 20.2%, and W: 15.2%. Electron beams with nominal energies of 4, 6, and 9 MeV generated with Clinac-iX (Varian Medical Systems., Palo Alto, CA, USA) were used. The water phantom QWP-06 (QualitA Co, Ltd., Nagano, Japan) and the water-equivalent phantom TM Phantom (Taisei Medical Co, Ltd., Osaka, Japan) were used for the measurements. The percentage depth ionization (PDI) for the thickness of TFP and the off-center
http://dx.doi.org/10.1016/j.ejmp.2014.05.009 1120-1797/© 2014 Associazione Italiana di Fisica Medica. Published by Elsevier Ltd. All rights reserved.
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ratio (OCR) for the size of the irradiation field formed by processing TFP were measured using parallel-plate ionization chambers and a film for dosimetry (Fig. 1). The dosimeter and film were NACP-02 (IBA Dosimetry, Schwarzenbruck, Germany) and EDR2 (Carestream Health, Inc., Rochester, USA), respectively. A lead shield (Shimadzu Co, Inc., Kyoto, Japan) was used as a reference material. Measurement of the transmission TFP was placed in the center of the space between the water phantom and the applicator of electron beams, and measurements were performed by changing the thickness of TFP and the size of the irradiation field. The TFP thickness was adjusted in advance to 0.0, 1.5, 3.0, 4.5, 6.0, 9.0, 12.0, and 15.0 mm, and the irradiation field size to 10 10 and 20 20 cm2. The PDI was obtained on a range from 0 to 8 cm in water phantom. The transmission at each TFP thickness was calculated in each case as the ratio between the ionization measured with and without shielding and compared according to the electron beam energy, irradiation field size, and measurement depth. Measurement of the dose profile TFP shaped to the size of the irradiation field was placed on the surface of the water-equivalent phantom, and the true size and flatness of the irradiation field were evaluated by measuring the dose profile using a film. The true irradiation field size was defined at the 95% dose level. The flatness was calculated as the ratio of the maximum dose relative to the dose at the field center. The thickness of TFP was 15.0 mm, and the size of the irradiation field was 8 2, 8 10, or 8 20 cm2. The film was placed at depths of the calibration point of the electron beam energy in the water-equivalent phantom of 0.6, 1.3, and 2.0 cm at 4, 6, and 9 MeV, respectively. For comparison with lead, a lead plate 3.0 mm thick was placed on the phantom surface, and the dose profile was measured similarly. The size of the irradiation field for this measurement was 8 9 cm2.
Figure 2. Relationship between TFP thickness and transmission for 10 10 cm2 field sizes with 4, 6, and 9 MeV electron energy. The horizontal axis is the TFP thickness, and the vertical axis is the transmission measured at the 0.5-cm buildup in water phantom. The transmission curves are normalized by the measurement value without TFP for each electron beam energy.
Results Measurement of the transmission Fig. 2 shows the transmission calculated from the measured PDI. The transmission in an irradiation field of 10 10 cm2 at TFP thicknesses of 6, 9, 12, and 15 mm were 0.5, 0.4, 0.4, and 0.3% at 4 MeV, 29.2, 4.6, 1.3, and 1.0% at 6 MeV, 79.5, 42.0, 17.5, and 5.8% at 9 MeV, respectively. Fig. 3 shows the transmission at various measurement depths in irradiation fields of 10 10 and 20 20 cm2 at 6 MeV. The transmission at a TFP thickness of 15 mm was 1.2 and 1.3% in the 10 10 and 20 20 cm2 irradiation fields, respectively. Table 1 shows the TFP thickness necessary to attenuate the transmission by 95 and 98%.
Position of TFP and surface dose in the shielded area Measurement of the dose profile In anticipation of the clinical use as a shield on a patient’s body surface, the surface dose was measured by placing TFP on and 2.5 cm away from the phantom surface. The thickness of TFP was 15.0 mm, and the size of the irradiation field was 10 10 or 20 20 cm2. Measurement was performed by placing the dosimeter on the phantom surface.
Fig. 4 shows the dose profiles at various energies in 3 irradiation fields with different window widths. The true irradiation field size of the 95% dose area was determined for each irradiation field size. It was 0.8, 9.2, and 13.8 cm at 4 MeV, 0.6, 9.0, and 19.4 cm at 6 MeV, and 0.8, 9.0, and 19.4 cm at 9 MeV for irradiation field sizes of 8 2,
Figure 1. Schematic diagram showing the setup of the experiment: (a) percentage depth ionization measurement for 10 10 cm2 and 20 20 cm2 field sizes with different thicknesses of TFP (0.0, 1.5, 3.0, 4.5, 6.0, 12.0, 15.0 mm), (b) off-center ratio measurement for 8 2, 10, 20 cm2 and 8 9 cm2 field sizes with 15 mm TFP or 3 mm lead, and (c) surface dose measurement with the location of TFP being 0.0 cm and 2.5 cm from the phantom surface.
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Discussion
Figure 3. Relationship between TFP thickness and transmission for (a) 10 10 cm2 and (b) 20 20 cm2 field sizes with electron energy of 6 MeV. The horizontal axis is the TFP thickness, and the vertical axis is the transmission measured at the surface, 0.5 cm with buildup and 1.5 cm without buildup in the water phantom. The transmission curves are normalized by the measurement value without TFP for each measured depth or field size.
8 10, and 8 20 cm2, respectively. In addition, the flatness of the irradiation field at various energy levels was 103%, 105.4, and 105.5% at 4, 6, and 9 MeV, respectively. Fig. 5 shows the results of comparison of the dose profiles between TFP and lead. When the irradiation field size was 8 9 cm2, the true irradiation field size of the 95% dose area was nearly equal between TFP and lead: 8.1 and 8.0 cm at 4 MeV, 7.6 and 7.7 cm at 6 MeV, and 7.8 and 7.6 cm at 9 MeV, respectively.
In a buildup region with a depth of 0.5 cm, the thicknesses of TFP that attenuate an electron beam by about 95% were 4, 9, and 15 mm at 4, 6, and 9 MeV, respectively. As shown in Table 1, the transmission of an electron beam with TFP shielding was dependent on the irradiation field size and the measurement depth. These results are in agreement with those reported by Parasad et al. [4], and similar dose characteristics are considered to be obtained with TFP and lead. In the measurement of the dose profile, the irradiation field size was nearly equal between TFP and lead. However, the dose profile of the irradiation field shaped with TFP showed gradual increases in the dose towards the periphery of the irradiation field. The peripheral dose relative to the central dose was largest at 9 MeV and showed an increase of about 5.5%. The increase in the dose in the periphery of the irradiation field was considered to be due to the effect of scattered rays from the shield material shaping the irradiation field and depended primarily on the thickness of TFP. The thickness of TFP needed to obtain an attenuation rate similar to that with 3.0-mm-thick lead is 15 mm or greater, although it also depends on the energy of the electron beam [10]. Therefore, scattered rays from the shield are considered to increase in the periphery of the irradiation field with slight deterioration of the flatness compared with that for lead. Clinically, bremsstrahlung due to interaction between the electron beam and the shield, and the bolus effect due to the shield itself pose problems [7]. Generally, bremsstrahlung is proportionate to the energy of the electron beam and the atomic number of the shield material. Since the atomic number of tungsten is lower than that of lead, bremsstrahlung may be reduced by using TFP. The transmission shown in Table 2 represents the dose on the skin surface with 15-mm TFP. The transmissions of electron beams at 4 and 6 MeV were 2% or less regardless of the position of TFP, and the effect of bremsstrahlung or the bolus effect is considered to be unremarkable in its clinical use. However, the transmission of the 9 MeV electron beam was about 10%, and further evaluations concerning the effects of the thickness of the shield and irradiation field size are necessary. Some materials have possibilities to be shielding material for electron beam or others. Yue et al. reported hydrogenated styreneebutadieneestyrene copolymer (SEBS) for electron beam shielding [12]. The shielding capability of SEBS including tungsten surpasses that of leads. The bilayer structure of SEBS and lead has more high shielding effect, although the shielding effect
Table 1 TFP thickness (mm) required for 95% and 98% reductions of each electron energy, field size, and measured depth. Energy (MeV) and field size (cm2)
Depth (cm)
Attenuation (%)
10 10
20 20
10 10
20 20
10 10
20 20
Position of TFP and surface dose in the shielded area
0.0
95 98
5.56 5.87
5.63 6.09
10.57 11.86
10.74 11.93
15.61a 16.20a
15.65a 16.18a
In the irradiation field of 10 10 cm2, the transit doses with TFP placed on the surface of the phantom and 2.5 cm away from the phantom were 0.5 and 0.4% at 4 MeV, 1.3 and 1.1% at 6 MeV, and 10.4 and 9.5% at 9 MeV, respectively. When the field size was 20 20 cm2, the transit dose increased by 0.2% with a maximum of 2.2% compared with that at 10 10 cm2 (Table 2).
0.5
95 98
4.31 5.11
4.30 5.08
8.88 11.14
8.94 11.34
15.06a 15.91a
15.20a 16.02a
1.5
95 98
0.98 1.33
0.98 1.34
5.67 7.63
5.70 7.91
11.49 15.12a
11.99 15.55a
3.0
95 98
e e
e e
e 0.66
e 0.77
4.99 8.63
5.49 11.47
a
4
6
9
Estimated value by linear interpolation with transmission curve.
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Figure 4. Comparison of beam profiles for 8 2 cm2, 8 10 cm2, 8 20 cm2 field sizes and electron energy of (a) 4 MeV, (b) 6 MeV, and (c) 9 MeV. The horizontal axis is the x-axis of the irradiated field, and the vertical axis is the relative dose normalized at the isocenter position. Beam profiles were measured at the peak depth for each electron energy in water-equivalent phantom.
of the bilayer structure depends on its stacking order. Tajiri et al. have proposed Rad-block consists of tungsten and some kinds of resin, including polyamide 6 for X-ray shielding [9]. Rad-block is easy to cut, and inexpensive to produce. Those materials have the same characteristics as to the extent of lead-free tungstenincluded materials and easy to produce. On the other hand,
tungsten distributions and flexibilities in materials are yet imprecision. TFP has heterogeneous tungsten distribution per unit area because of its thin thickness. Heterogeneous area, however, decrease because shielding materials are ordinary deposited layer upon layer. It leads shielding materials to be homogeneous materials.
Figure 5. Comparison of beam profiles for TFP and lead and electron energies of (a) 6 MeV and (b) 9 MeV. The horizontal axis is the x-axis of the irradiated field (8 9 cm2), and the vertical axis is the relative dose normalized at the isocenter position.
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Table 2 Transmission (%) with distance of TFP (15 mm thickness) from the water-equivalent phantom surface for 10 10 cm2 and 20 20 cm2 field sizes and each electron energy. Distance from surface (cm)
0 2.5
Energy (MeV) and field size (cm2) 4
6
9
Conflict of interest statement The authors report no conflicts of interest in conducting the research. Acknowledgement
10 10
20 20
10 10
20 20
10 10
20 20
0.5 0.4
0.6 0.5
1.3 1.1
1.5 1.3
10.4 9.5
10.6 11.7
A characteristic of TFP is that it is a paper-like shield. Since it can be readily processed by cutting and attaching similarly to paper, it can be applied to targets with a variety of shapes. While the cost of TFP is slightly higher than that of lead of the same size, this may be reduced by reuse. It is also ecologically advantageous as it can be recycled after use as tungsten and paper. However, the shielding performance of a sheet of TFP is low, and a large number of sheets must be layered for clinical use. For the future, it is necessary to improve its shielding ability or to modify the sheet thickness. We are also considering determination of the metal powder type, filling rate, and mass attenuation coefficient appropriate for shielding electron beams using Monte Carlo simulations.
Conclusion In this study, the shielding characteristics of TFP against electron beams were evaluated. The transmission of shielded electron beams was shown to depend on the measurement depth and irradiation field size. In addition, a high radiation-shielding effect was found to be obtained depending on the energy and TFP thickness used. TFP with novel characteristics and excellent processability is highly useful as a shielding material, and its potential for further clinical application was suggested.
This research was presented at the poster session of the European Cancer Congress in Amsterdam from September 27theOctober 1st, 2013. This work was supported by JSPS KAKENHI grant number 25461877. The study sponsor had no involvement in the study design, or in the collection, analysis, or interpretation of data. References [1] Giarratano JC, Duerkes RJ, Almond PR. Lead shielding thickness for dose reduction of 7- to 28 MeV electrons. Med Phys 1975;2:336e7. [2] Khan FM, Moore VC, Levitt SH. Field shaping in electron beam therapy. Br J Radiol 1976;49:883e6. [3] Khan FM, Werner BL, Deibel FC. Lead shielding for electrons. Med Phys 1981;8:712e3. [4] Prasad SG, Parthasaradhi K, Arbetter S, Lee Y, Garces R. Lead shielding thickness for dose reduction of 6-MeV electrons for different square fields. Med Phys 1988;15:263e6. [5] Prasad SG, Parthasaradhi K, Lee Y, Garces R. Lead shielding thickness for dose reduction of 5 MeV electrons. Med Phys 1989;16:807e8. [6] Stewart RJ, Dredge TJ, Langenegger A, Wongse-Ek C, Karolis C, Oliver LD. Lead shielding for electron beams from 6-18 MeV. Austral Radiol 1983;27:73e8. [7] Shiomoto A, Akazawa H, Okada T. Evaluation of radiation therapy for keloid using electron beam. Nippon Hoshasen Gijutsu Gakkai Zasshi (Jpn J Radiol Technol) 2003;60:429e36. [8] Needleman H. Lead poisoning. Annu Rev Med 2004;55:209e22. [9] Tajiri M, Sunaoka M, Fukumura A, Endo M. A new radiation shielding block material for radiation therapy. Med Phys 2004;31:3022e3. [10] Verhaegen F, Buffa FM, Deehan C. Quantifying effects of lead shielding in electron beams: a Monte Carlo study. Phys Med Biol 2001;46:757e69. [11] Weaver RD, Gerbi BJ, Dusenbery KE. Evaluation of eye shields made of tungsten and aluminum in high-energy electron beams. Int J Radiat Oncol Biol Phys 1998;41:233e7. [12] Yue K, Luo W, Dong X, Wang C, Wu G, Jiang M, et al. A new lead-free radiation shielding material for radiotherapy. Radiat Prot Dosimetry 2009;133:256e60. [13] Yue K, Yao Y, Dong X, Luo W. A novel shielding scheme studied by the Monte Carlo method for electron beam radiotherapy. Health Phys 2013;104:277e81.
Contents lists available at ScienceDirect
Physica Medica journal homepage: http://www.physicamedica.com
Technical notes
Dosimetric shield evaluation with tungsten sheet in 4, 6, and 9 MeV electron beams Takahiro Fujimoto a, Hajime Monzen b, *, Manabu Nakata a, Takashi Okada a, Shinsuke Yano a, Toru Takakura a, Junichi Kuwahara a, Makoto Sasaki a, Kyoji Higashimura a, Masahiro Hiraoka b a
Clinical Radiology Service Division, Kyoto University Hospital, Kyoto 606-8397, Japan Department of Radiation Oncology and Image-Applied Therapy, Kyoto University Graduate School of Medicine, 54 Kawara-cho, Shogoin, Sakyo-ku, Kyoto 606-8507, Japan
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 February 2014 Received in revised form 25 May 2014 Accepted 27 May 2014 Available online 19 June 2014
In electron radiotherapy, shielding material is required to attenuate beam and scatter. A newly introduced shielding material, tungsten functional paper (TFP), has been anticipated to become a very useful device that is lead-free, light, flexible, and easily processed, containing very fine tungsten powder at as much as 80% by weight. The purpose of this study was to investigate the dosimetric changes due to TFP shielding for electron beams. TFP (thickness 0e15 mm) was placed on water or a water-equivalent phantom. Percentage depth ionization and transmission were measured for 4, 6, and 9 MeV electron beams. Off-center ratio was also measured using film dosimetry at depth of dose maximum under similar conditions. Then, beam profiles and transmission with two shielding materials, TFP and lead, were evaluated. Reductions of 95% by using TFP at 0.5 cm depth occurred at 4, 9, and 15 mm with 4, 6, and 9 MeV electron beams, respectively. It is found that the dose tend to increase at the field edge shaped with TFP, which might be influenced by the thickness. TFP has several unique features and is very promising as a useful tool for radiation protection for electron beams, among others. © 2014 Associazione Italiana di Fisica Medica. Published by Elsevier Ltd. All rights reserved.
Keywords: Tungsten paper Electron beam Transmission Radiation shield
Introduction In electron radiotherapy, shields are placed above the patient's body surface to block beams and scattered rays. By appropriately shaping the shield, which is most commonly made of lead or lowmelting point alloy (LMA), radiation can be concentrated to the appropriate area by forming an irradiation field. There have been a number of reports on the dose characteristics of electron beams with lead shields, and the dose is considered to depend on the energy, size of the irradiation field, and thickness of the shield material [1e7]. Lead has excellent shielding properties against electron beams and is available at a reasonable price. However, its effects on the environment and toxicity to the human body have been identified as problems [8]. While there have been reports on the use of elements with a high atomic number, such as tungsten, as substitutes for lead [9e13], they are not commonly used owing to the difficulty in processing and their high cost. Recently, rubber or
* Corresponding author. Tel.: þ81 751 3762; fax: þ81 771 9749. E-mail address: [email protected] (H. Monzen).
plastic containing tungsten powder, which has advantages including easy processability, has been developed. In this study, the shielding performance of tungsten functional paper (TFP) newly developed as a shielding material against electron beams was evaluated. The energy of electron beams and changes in their dose characteristics with the size of the irradiation field were also measured to evaluate the possibility of its clinical use. Materials and methods TFP prepared by Toppan Printing Co., Ltd., is a sheet-form shielding material 0.3 mm in thickness containing tungsten powder at about 80% by weight. The element ratios of TFP (mol%) are H: 24.2%, C: 40.4%, O: 20.2%, and W: 15.2%. Electron beams with nominal energies of 4, 6, and 9 MeV generated with Clinac-iX (Varian Medical Systems., Palo Alto, CA, USA) were used. The water phantom QWP-06 (QualitA Co, Ltd., Nagano, Japan) and the water-equivalent phantom TM Phantom (Taisei Medical Co, Ltd., Osaka, Japan) were used for the measurements. The percentage depth ionization (PDI) for the thickness of TFP and the off-center
http://dx.doi.org/10.1016/j.ejmp.2014.05.009 1120-1797/© 2014 Associazione Italiana di Fisica Medica. Published by Elsevier Ltd. All rights reserved.
T. Fujimoto et al. / Physica Medica 30 (2014) 838e842
839
ratio (OCR) for the size of the irradiation field formed by processing TFP were measured using parallel-plate ionization chambers and a film for dosimetry (Fig. 1). The dosimeter and film were NACP-02 (IBA Dosimetry, Schwarzenbruck, Germany) and EDR2 (Carestream Health, Inc., Rochester, USA), respectively. A lead shield (Shimadzu Co, Inc., Kyoto, Japan) was used as a reference material. Measurement of the transmission TFP was placed in the center of the space between the water phantom and the applicator of electron beams, and measurements were performed by changing the thickness of TFP and the size of the irradiation field. The TFP thickness was adjusted in advance to 0.0, 1.5, 3.0, 4.5, 6.0, 9.0, 12.0, and 15.0 mm, and the irradiation field size to 10 10 and 20 20 cm2. The PDI was obtained on a range from 0 to 8 cm in water phantom. The transmission at each TFP thickness was calculated in each case as the ratio between the ionization measured with and without shielding and compared according to the electron beam energy, irradiation field size, and measurement depth. Measurement of the dose profile TFP shaped to the size of the irradiation field was placed on the surface of the water-equivalent phantom, and the true size and flatness of the irradiation field were evaluated by measuring the dose profile using a film. The true irradiation field size was defined at the 95% dose level. The flatness was calculated as the ratio of the maximum dose relative to the dose at the field center. The thickness of TFP was 15.0 mm, and the size of the irradiation field was 8 2, 8 10, or 8 20 cm2. The film was placed at depths of the calibration point of the electron beam energy in the water-equivalent phantom of 0.6, 1.3, and 2.0 cm at 4, 6, and 9 MeV, respectively. For comparison with lead, a lead plate 3.0 mm thick was placed on the phantom surface, and the dose profile was measured similarly. The size of the irradiation field for this measurement was 8 9 cm2.
Figure 2. Relationship between TFP thickness and transmission for 10 10 cm2 field sizes with 4, 6, and 9 MeV electron energy. The horizontal axis is the TFP thickness, and the vertical axis is the transmission measured at the 0.5-cm buildup in water phantom. The transmission curves are normalized by the measurement value without TFP for each electron beam energy.
Results Measurement of the transmission Fig. 2 shows the transmission calculated from the measured PDI. The transmission in an irradiation field of 10 10 cm2 at TFP thicknesses of 6, 9, 12, and 15 mm were 0.5, 0.4, 0.4, and 0.3% at 4 MeV, 29.2, 4.6, 1.3, and 1.0% at 6 MeV, 79.5, 42.0, 17.5, and 5.8% at 9 MeV, respectively. Fig. 3 shows the transmission at various measurement depths in irradiation fields of 10 10 and 20 20 cm2 at 6 MeV. The transmission at a TFP thickness of 15 mm was 1.2 and 1.3% in the 10 10 and 20 20 cm2 irradiation fields, respectively. Table 1 shows the TFP thickness necessary to attenuate the transmission by 95 and 98%.
Position of TFP and surface dose in the shielded area Measurement of the dose profile In anticipation of the clinical use as a shield on a patient’s body surface, the surface dose was measured by placing TFP on and 2.5 cm away from the phantom surface. The thickness of TFP was 15.0 mm, and the size of the irradiation field was 10 10 or 20 20 cm2. Measurement was performed by placing the dosimeter on the phantom surface.
Fig. 4 shows the dose profiles at various energies in 3 irradiation fields with different window widths. The true irradiation field size of the 95% dose area was determined for each irradiation field size. It was 0.8, 9.2, and 13.8 cm at 4 MeV, 0.6, 9.0, and 19.4 cm at 6 MeV, and 0.8, 9.0, and 19.4 cm at 9 MeV for irradiation field sizes of 8 2,
Figure 1. Schematic diagram showing the setup of the experiment: (a) percentage depth ionization measurement for 10 10 cm2 and 20 20 cm2 field sizes with different thicknesses of TFP (0.0, 1.5, 3.0, 4.5, 6.0, 12.0, 15.0 mm), (b) off-center ratio measurement for 8 2, 10, 20 cm2 and 8 9 cm2 field sizes with 15 mm TFP or 3 mm lead, and (c) surface dose measurement with the location of TFP being 0.0 cm and 2.5 cm from the phantom surface.
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Discussion
Figure 3. Relationship between TFP thickness and transmission for (a) 10 10 cm2 and (b) 20 20 cm2 field sizes with electron energy of 6 MeV. The horizontal axis is the TFP thickness, and the vertical axis is the transmission measured at the surface, 0.5 cm with buildup and 1.5 cm without buildup in the water phantom. The transmission curves are normalized by the measurement value without TFP for each measured depth or field size.
8 10, and 8 20 cm2, respectively. In addition, the flatness of the irradiation field at various energy levels was 103%, 105.4, and 105.5% at 4, 6, and 9 MeV, respectively. Fig. 5 shows the results of comparison of the dose profiles between TFP and lead. When the irradiation field size was 8 9 cm2, the true irradiation field size of the 95% dose area was nearly equal between TFP and lead: 8.1 and 8.0 cm at 4 MeV, 7.6 and 7.7 cm at 6 MeV, and 7.8 and 7.6 cm at 9 MeV, respectively.
In a buildup region with a depth of 0.5 cm, the thicknesses of TFP that attenuate an electron beam by about 95% were 4, 9, and 15 mm at 4, 6, and 9 MeV, respectively. As shown in Table 1, the transmission of an electron beam with TFP shielding was dependent on the irradiation field size and the measurement depth. These results are in agreement with those reported by Parasad et al. [4], and similar dose characteristics are considered to be obtained with TFP and lead. In the measurement of the dose profile, the irradiation field size was nearly equal between TFP and lead. However, the dose profile of the irradiation field shaped with TFP showed gradual increases in the dose towards the periphery of the irradiation field. The peripheral dose relative to the central dose was largest at 9 MeV and showed an increase of about 5.5%. The increase in the dose in the periphery of the irradiation field was considered to be due to the effect of scattered rays from the shield material shaping the irradiation field and depended primarily on the thickness of TFP. The thickness of TFP needed to obtain an attenuation rate similar to that with 3.0-mm-thick lead is 15 mm or greater, although it also depends on the energy of the electron beam [10]. Therefore, scattered rays from the shield are considered to increase in the periphery of the irradiation field with slight deterioration of the flatness compared with that for lead. Clinically, bremsstrahlung due to interaction between the electron beam and the shield, and the bolus effect due to the shield itself pose problems [7]. Generally, bremsstrahlung is proportionate to the energy of the electron beam and the atomic number of the shield material. Since the atomic number of tungsten is lower than that of lead, bremsstrahlung may be reduced by using TFP. The transmission shown in Table 2 represents the dose on the skin surface with 15-mm TFP. The transmissions of electron beams at 4 and 6 MeV were 2% or less regardless of the position of TFP, and the effect of bremsstrahlung or the bolus effect is considered to be unremarkable in its clinical use. However, the transmission of the 9 MeV electron beam was about 10%, and further evaluations concerning the effects of the thickness of the shield and irradiation field size are necessary. Some materials have possibilities to be shielding material for electron beam or others. Yue et al. reported hydrogenated styreneebutadieneestyrene copolymer (SEBS) for electron beam shielding [12]. The shielding capability of SEBS including tungsten surpasses that of leads. The bilayer structure of SEBS and lead has more high shielding effect, although the shielding effect
Table 1 TFP thickness (mm) required for 95% and 98% reductions of each electron energy, field size, and measured depth. Energy (MeV) and field size (cm2)
Depth (cm)
Attenuation (%)
10 10
20 20
10 10
20 20
10 10
20 20
Position of TFP and surface dose in the shielded area
0.0
95 98
5.56 5.87
5.63 6.09
10.57 11.86
10.74 11.93
15.61a 16.20a
15.65a 16.18a
In the irradiation field of 10 10 cm2, the transit doses with TFP placed on the surface of the phantom and 2.5 cm away from the phantom were 0.5 and 0.4% at 4 MeV, 1.3 and 1.1% at 6 MeV, and 10.4 and 9.5% at 9 MeV, respectively. When the field size was 20 20 cm2, the transit dose increased by 0.2% with a maximum of 2.2% compared with that at 10 10 cm2 (Table 2).
0.5
95 98
4.31 5.11
4.30 5.08
8.88 11.14
8.94 11.34
15.06a 15.91a
15.20a 16.02a
1.5
95 98
0.98 1.33
0.98 1.34
5.67 7.63
5.70 7.91
11.49 15.12a
11.99 15.55a
3.0
95 98
e e
e e
e 0.66
e 0.77
4.99 8.63
5.49 11.47
a
4
6
9
Estimated value by linear interpolation with transmission curve.
T. Fujimoto et al. / Physica Medica 30 (2014) 838e842
841
Figure 4. Comparison of beam profiles for 8 2 cm2, 8 10 cm2, 8 20 cm2 field sizes and electron energy of (a) 4 MeV, (b) 6 MeV, and (c) 9 MeV. The horizontal axis is the x-axis of the irradiated field, and the vertical axis is the relative dose normalized at the isocenter position. Beam profiles were measured at the peak depth for each electron energy in water-equivalent phantom.
of the bilayer structure depends on its stacking order. Tajiri et al. have proposed Rad-block consists of tungsten and some kinds of resin, including polyamide 6 for X-ray shielding [9]. Rad-block is easy to cut, and inexpensive to produce. Those materials have the same characteristics as to the extent of lead-free tungstenincluded materials and easy to produce. On the other hand,
tungsten distributions and flexibilities in materials are yet imprecision. TFP has heterogeneous tungsten distribution per unit area because of its thin thickness. Heterogeneous area, however, decrease because shielding materials are ordinary deposited layer upon layer. It leads shielding materials to be homogeneous materials.
Figure 5. Comparison of beam profiles for TFP and lead and electron energies of (a) 6 MeV and (b) 9 MeV. The horizontal axis is the x-axis of the irradiated field (8 9 cm2), and the vertical axis is the relative dose normalized at the isocenter position.
842
T. Fujimoto et al. / Physica Medica 30 (2014) 838e842
Table 2 Transmission (%) with distance of TFP (15 mm thickness) from the water-equivalent phantom surface for 10 10 cm2 and 20 20 cm2 field sizes and each electron energy. Distance from surface (cm)
0 2.5
Energy (MeV) and field size (cm2) 4
6
9
Conflict of interest statement The authors report no conflicts of interest in conducting the research. Acknowledgement
10 10
20 20
10 10
20 20
10 10
20 20
0.5 0.4
0.6 0.5
1.3 1.1
1.5 1.3
10.4 9.5
10.6 11.7
A characteristic of TFP is that it is a paper-like shield. Since it can be readily processed by cutting and attaching similarly to paper, it can be applied to targets with a variety of shapes. While the cost of TFP is slightly higher than that of lead of the same size, this may be reduced by reuse. It is also ecologically advantageous as it can be recycled after use as tungsten and paper. However, the shielding performance of a sheet of TFP is low, and a large number of sheets must be layered for clinical use. For the future, it is necessary to improve its shielding ability or to modify the sheet thickness. We are also considering determination of the metal powder type, filling rate, and mass attenuation coefficient appropriate for shielding electron beams using Monte Carlo simulations.
Conclusion In this study, the shielding characteristics of TFP against electron beams were evaluated. The transmission of shielded electron beams was shown to depend on the measurement depth and irradiation field size. In addition, a high radiation-shielding effect was found to be obtained depending on the energy and TFP thickness used. TFP with novel characteristics and excellent processability is highly useful as a shielding material, and its potential for further clinical application was suggested.
This research was presented at the poster session of the European Cancer Congress in Amsterdam from September 27theOctober 1st, 2013. This work was supported by JSPS KAKENHI grant number 25461877. The study sponsor had no involvement in the study design, or in the collection, analysis, or interpretation of data. References [1] Giarratano JC, Duerkes RJ, Almond PR. Lead shielding thickness for dose reduction of 7- to 28 MeV electrons. Med Phys 1975;2:336e7. [2] Khan FM, Moore VC, Levitt SH. Field shaping in electron beam therapy. Br J Radiol 1976;49:883e6. [3] Khan FM, Werner BL, Deibel FC. Lead shielding for electrons. Med Phys 1981;8:712e3. [4] Prasad SG, Parthasaradhi K, Arbetter S, Lee Y, Garces R. Lead shielding thickness for dose reduction of 6-MeV electrons for different square fields. Med Phys 1988;15:263e6. [5] Prasad SG, Parthasaradhi K, Lee Y, Garces R. Lead shielding thickness for dose reduction of 5 MeV electrons. Med Phys 1989;16:807e8. [6] Stewart RJ, Dredge TJ, Langenegger A, Wongse-Ek C, Karolis C, Oliver LD. Lead shielding for electron beams from 6-18 MeV. Austral Radiol 1983;27:73e8. [7] Shiomoto A, Akazawa H, Okada T. Evaluation of radiation therapy for keloid using electron beam. Nippon Hoshasen Gijutsu Gakkai Zasshi (Jpn J Radiol Technol) 2003;60:429e36. [8] Needleman H. Lead poisoning. Annu Rev Med 2004;55:209e22. [9] Tajiri M, Sunaoka M, Fukumura A, Endo M. A new radiation shielding block material for radiation therapy. Med Phys 2004;31:3022e3. [10] Verhaegen F, Buffa FM, Deehan C. Quantifying effects of lead shielding in electron beams: a Monte Carlo study. Phys Med Biol 2001;46:757e69. [11] Weaver RD, Gerbi BJ, Dusenbery KE. Evaluation of eye shields made of tungsten and aluminum in high-energy electron beams. Int J Radiat Oncol Biol Phys 1998;41:233e7. [12] Yue K, Luo W, Dong X, Wang C, Wu G, Jiang M, et al. A new lead-free radiation shielding material for radiotherapy. Radiat Prot Dosimetry 2009;133:256e60. [13] Yue K, Yao Y, Dong X, Luo W. A novel shielding scheme studied by the Monte Carlo method for electron beam radiotherapy. Health Phys 2013;104:277e81.