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Evaluation of two water-equivalent phantom materials for output calibration of photon and electron beams
Medical Dosimetry, Vol. 28, No. 4, pp. 267⫺269, 2003 Copyright © 2003 American Association of Medical Dosimetrists Printed in the USA. All rights reserved 0958-3947/03/$–see front matter
doi:10.1016/j.meddos.2003.09.001
EVALUATION OF TWO WATER-EQUIVALENT PHANTOM MATERIALS FOR OUTPUT CALIBRATION OF PHOTON AND ELECTRON BEAMS LIZHONG LIU, SATISH C. PRASAD, and DANIEL A. BASSANO Department of Radiation Oncology, SUNY Upstate Medical University, Syracuse, NY (Received 30 March 2003; accepted 10 September 2003)
Abstract—Two commercially available water-equivalent solid phantom materials were evaluated for output calibration in both photon (6 –15 MV) and electron (6 –20 MeV) beams. The solid water 457 and virtual water materials have the same chemical composition but differ in manufacturing process and density. A Farmer-type ionization chamber was used for measuring the output of the photon beams at 5- and 10-cm depth and electron beams at maximum buildup depth in the solid phantoms and in natural water. The water-equivalency correction factor for the solid materials is defined as the ratio of the chamber reading in natural water to that in the solid at the same linear depth. For photon beams, the correction factor was found to be independent of depth and was 0.987 and 0.993 for 6- and 15-MV beams, respectively, for solid water. For virtual water, the corresponding correction factors were 0.993 and 0.998 for 6- and 15-MV beams, respectively. For electron beams, the correction factors ranged from 1.013 to 1.007 for energies of 6 to 20 MeV for both solid materials. This indicated that the water-equivalency of these materials is within ⴞ 1.3%, making them suitable substitutes for natural water in both photon and electron beam output measurements over a wide energy range. These correction factors are slightly larger than the manufacturers’ advertised values (ⴞ 1.0% for solid water and ⴞ 0.5% for virtual water). We suggest that these corrections are large enough in most cases and should be applied in the calculation of beam outputs. © 2003 American Association of Medical Dosimetrists. Key Words: Solid phantom, Water equivalency, Beam calibration.
solid water, but differs in manufacturing process and density, and is claimed by the manufacturer to have improved water equivalency. Although some of these solid materials were independently verified5,6,8 to be suitable as water substitute under certain conditions, the degree of water equivalency for a given material8 was found to be dependent on the measurement technique, modality, and beam energy. Both the value and variation of the water equivalency were found to be as much as 2.5%. Such a range was wide enough that careful comparison with natural water was recommended8 before a solid phantom could be used for clinical beam output calibration or constancy checks. In this study, we examine the water equivalency of the solid water and the recently developed virtual water materials for output measurements of high-energy photon and electron beams. The measurements were done in 6- and 15-MV photon beams at several depths, including the one recommended by the recently published AAPM TG-51 protocol,2 and at the depths of maximum dose for 6- to 20-MeV electrons. The water equivalency was investigated by intercomparison of the beam output measurements in solid phantom and natural water. Improved water equivalency was observed for virtual water compared to solid water for photon beams. Both the solid water and the virtual water were found to be suitable substitutes for natural water for both photon and electron beam output measurements. The water equivalency for both materials was within 1.3% or less, indicating that
INTRODUCTION All modern radiotherapy beam calibration protocols use water as the standard medium for dose determination.1,2 The primary reasons for this choice are multiple-fold: water is universally available, its physical properties are homogenous and uniform, and its beam absorption properties closely match that of soft tissues. It is, however, time consuming to setup a water tank, and measurement equipment, i.e., ion chambers, have to be waterproof. Over the last 2 decades, extensive efforts have been directed at developing solid phantom materials as substitute for natural water. This has lead to such commercial products as solid water (GAMMEX RMI, Middleton, WI); plastic water (Computerized Imaging References Systems, Inc., Norfolk, VA); and white water (PTW-Freiburgh, Freiburg, Germany). In particular, solid water has emerged as a leading candidate as a water-equivalent phantom material for both photon and electron beam output measurements.3– 6 In addition, solid water is also widely used as a phantom material for dose characterization of low-energy brachytherapy sources.7 Recently, a new solid phantom material, virtual water, was introduced by MED-TEC, Inc. (Orange City, IA.). The new material has the same chemical composition as Reprint requests to: Lizhong Liu, Ph.D., Department of Radiation Oncology, SUNY Upstate Medical University, 750 East Adams Street, Syracuse, NY 13210. E-mail: [email protected] This work was presented at the 2000 AAPM Annual Meeting in Chicago, IL 267
268
Medical Dosimetry
Table 1. Physical properties of solid water and virtual water phantom materials Nominal Density Measured Density (g/cm3) (g/cm3)
Material Solid Water 457 Virtual Water
1.015 1.03
Manufacturer
1.042 (⫾ 2%) GAMMEX RMI 1.070 (⫾ 1%) MED-TEC, Inc.
Volume 28, Number 4, 2003
Table 2. Water-equivalency correction factor for photon beams Photon Energy (MV) 6 15
correction factors should be applied in the calculation of beam outputs. MATERIALS AND METHODS The physical properties of the 2 solid phantom materials used in this work are listed in Table 1. Both the solid water model 457 and the virtual water are resins having the same chemical composition; however, the manufacturing processes differ, resulting in slightly different density and consistency. Both materials were designed for use in high-energy photon and electron beams. In our measurements, all phantom materials consisted of 30-cm-square slabs of various thickness, ranging from 0.2 cm, 0.5 cm, 1 cm, 2 cm, 5 cm, to 6 cm. The density of each slab was determined by measuring the weight and thickness using a weight scale and a micrometer, respectively. The measured density and its spread among the slabs are listed in Table 1. These values differ from the nominal density provided by the manufacturers by up to 4%. For each phantom material in our measurements, the 6-cm-thick slab had a cavity drilled out at the depth of 1 cm to accommodate an NE2571 (NE Technology Limited, Berkshire, England.) 0.6-cc Farmer-type ion chamber. The water tank used in this study was a Lucite box of 20-cm square by 30-cm high. Waterproofing of the NE2571 Farmer chamber was achieved using a 1-mmthick Lucite sleeve that fitted nicely on the ion chamber. A portable sealed water phantom with chamber cavity at 5-cm depth was also used for some of the measurements in photons. High-energy photon and electron beams were provided by a Varian 2100C linear accelerator (Varian Medical Systems, Palo Alto, CA). Depths of 5 cm and 10 cm were used for the measurements with 6- and 15-MV photon beams. For electron beams, depths at the maximum dose buildup were used for 5 electron beams at 6, 9, 12, 16, and 20 MeV. Measurements in both the solid materials and the water phantom were at the same linear depth, using the same ion chamber and electrometer system. A water-equivalency correction factor (WECF) is defined to quantify the degree of water equivalency for the solid phantom material. This factor is simply the ratio of the ion chamber charge reading in the water phantom to that in the solid material phantom at the same linear depth, i.e., WECF ⫽ (electrometer reading in water)/ (electrometer reading in solid material).
Depth (cm)
WECFsolid water
WECFvirtual water
5 10 5 10
0.986 0.988 0.993 0.992
0.992 0.993 0.998 0.998
RESULTS AND DISCUSSION The water-equivalency correction factors for the 6and 15-MV photon beams are shown in Table 2. No depth dependence between 5 and 10 cm was observed for both photon energies in both solid materials. The corrections for the 6-MV beam were slightly larger than the 15-MV beam for both materials. The newly developed virtual water material indeed showed improved water equivalency for photons with a maximum correction of 0.8%, as compared to 1.4% for the solid water. This was in general agreement with claims made by the manufacturer, but was slightly outside of the 0.5% range advertised. The water equivalency correction factors for the electron beams are shown in Table 3. Unlike the photon beams, no noticeable differences in the correction factors between the 2 solid materials were observed for the electron beams. Therefore, the newly developed virtual water had improved water equivalency in photon beams only. The correction factors for the 2 solid materials display a weak dependence on the electron energy, with a larger correction for the lower energies. The maximum correction was about 1.4% for both materials. It was also interesting to note that the correction factors in electron beams were always greater than unity, whereas those in photon beams were less than unity. The magnitudes of the corrections in both cases were, however, comparable, indicating that the chemical compositions of the 2 materials were optimized among all beam modalities and energies. Solid water model 457 was investigated in one previous study8 using several different measurement techniques at different linear (cm) or mass (g/cm2) depths, and with or without depth dose or depth ionization corrections in solid. Our measurement technique of using linear depth without depth dose or ionization corTable 3. Water-equivalency correction factor for electron beams Electron Energy (MeV)
Depth (cm)
WECFsolid water
WECFvirtual water
6 9 12 16 20
1.5 2.2 2.5 2.5 2.5
1.011 1.013 1.011 1.008 1.007
1.012 1.014 1.008 1.007 1.008
Water-equivalent phantom materials ● L. LIU et al.
rection corresponds to the techniques IV and I, respectively, for the photon and electron beams in Ref. 8. The authors of Ref. 8 reported that for photon beams from Cobalt-60 (60Co) to 24 MV, the ratio of dose in solid water materials over the dose in water varied from 1.009 to 0.989. This corresponded to a water-equivalency correction factor of 0.991 to 1.011, because the WECF defined in our study was essentially the inverse of the dose ratio. Excluding the lower and higher energies and the different linear accelerator models narrowed the spread of the correction factor to 0.992 to 1.002 for 6- to 15-MV photons. This was comparable with the range of 0.986 to 0.993 observed in this study for the same material (see Table 2). However, the 2 ranges appeared slightly shifted relative to each other by 0.6% to 0.9%, due perhaps to inconsistencies among phantom slabs from different batches. For electron beams of 6 to 15 MeV, a dose ratio of 0.985 to 0.99 (i.e., a WECF factor of 1.015 to 1.01) was reported in Ref. 8. This was in excellent agreement with our result of 1.013 to 1.007 for the same material (see Table 3). CONCLUSIONS Two commercially-available solid phantom materials were evaluated for their water equivalency in output calibration of photon and electron beams. The maximum differences in outputs measured in the 2 solid materials compared to water were found to be 1.4%, making both the solid water and the newly developed virtual water suitable substitutes for natural water in photon and elec-
269
tron beam output measurements. The virtual water was found to have improved water equivalency in photons as compared to solid water, however, the response in electrons were identical for both materials. The magnitudes of the correction, however, were not negligibly small, and the water equivalency correction factor should be included in the output calculation. These results indicate that any phantom material intended for beam output measurements should undergo a quality assurance test before being put into clinical use. REFERENCES 1. AAPM TG-21. A protocol for the determination of absorbed dose from high-energy photon and electron beams. Med. Phys. 10:741– 71; 1983. Erratum: A protocol for the determination of absorbed dose from high-energy photon and electron beams. Med. Phys. 11:213; 1984. 2. AAPM TG-51. AAPM’s TG-51 protocol for clinical reference dosimetry of high-energy photon and electron beams. Med. Phys. 26:1847–70; 1999. 3. Constantinou, C.; Attix, F.; Paliwal, B. A solid water phantom material for radiotherapy x-ray and x-ray beam calibrations. Med. Phys. 9:436 –41; 1982. 4. Ho, A.; Paliwal, B. Stopping-power and mass energy-absorption coefficient ratios for solid water. Med. Phys. 31:403–4; 1986. 5. Thomadsen, B.; Constantinou, C.; Ho, A. Evaluation of waterequivalent plastics as phantom material for electron beam dosimetry. Med. Phys. 22:291; 1995. 6. Prasad, S.C. Comparison of absorbed doses in water and solid water for electron beams. Med. Dosim. 17:205–6; 1992. 7. Meigooni, A.G.; Li, Z.; Mishra, V.; Williamson, J.F. A comparative study of dosimetric properties of plastic water and solid water in brachytherapy applications. Med. Phys. 21:1983–7; 1994. 8. Tello, V.M.; Tailor, R.C.; Hanson, W.F. How water equivalent are water-equivalent solid materials for output calibration of photon and electron beams? Med. Phys. 22:1177–89; 1995.
doi:10.1016/j.meddos.2003.09.001
EVALUATION OF TWO WATER-EQUIVALENT PHANTOM MATERIALS FOR OUTPUT CALIBRATION OF PHOTON AND ELECTRON BEAMS LIZHONG LIU, SATISH C. PRASAD, and DANIEL A. BASSANO Department of Radiation Oncology, SUNY Upstate Medical University, Syracuse, NY (Received 30 March 2003; accepted 10 September 2003)
Abstract—Two commercially available water-equivalent solid phantom materials were evaluated for output calibration in both photon (6 –15 MV) and electron (6 –20 MeV) beams. The solid water 457 and virtual water materials have the same chemical composition but differ in manufacturing process and density. A Farmer-type ionization chamber was used for measuring the output of the photon beams at 5- and 10-cm depth and electron beams at maximum buildup depth in the solid phantoms and in natural water. The water-equivalency correction factor for the solid materials is defined as the ratio of the chamber reading in natural water to that in the solid at the same linear depth. For photon beams, the correction factor was found to be independent of depth and was 0.987 and 0.993 for 6- and 15-MV beams, respectively, for solid water. For virtual water, the corresponding correction factors were 0.993 and 0.998 for 6- and 15-MV beams, respectively. For electron beams, the correction factors ranged from 1.013 to 1.007 for energies of 6 to 20 MeV for both solid materials. This indicated that the water-equivalency of these materials is within ⴞ 1.3%, making them suitable substitutes for natural water in both photon and electron beam output measurements over a wide energy range. These correction factors are slightly larger than the manufacturers’ advertised values (ⴞ 1.0% for solid water and ⴞ 0.5% for virtual water). We suggest that these corrections are large enough in most cases and should be applied in the calculation of beam outputs. © 2003 American Association of Medical Dosimetrists. Key Words: Solid phantom, Water equivalency, Beam calibration.
solid water, but differs in manufacturing process and density, and is claimed by the manufacturer to have improved water equivalency. Although some of these solid materials were independently verified5,6,8 to be suitable as water substitute under certain conditions, the degree of water equivalency for a given material8 was found to be dependent on the measurement technique, modality, and beam energy. Both the value and variation of the water equivalency were found to be as much as 2.5%. Such a range was wide enough that careful comparison with natural water was recommended8 before a solid phantom could be used for clinical beam output calibration or constancy checks. In this study, we examine the water equivalency of the solid water and the recently developed virtual water materials for output measurements of high-energy photon and electron beams. The measurements were done in 6- and 15-MV photon beams at several depths, including the one recommended by the recently published AAPM TG-51 protocol,2 and at the depths of maximum dose for 6- to 20-MeV electrons. The water equivalency was investigated by intercomparison of the beam output measurements in solid phantom and natural water. Improved water equivalency was observed for virtual water compared to solid water for photon beams. Both the solid water and the virtual water were found to be suitable substitutes for natural water for both photon and electron beam output measurements. The water equivalency for both materials was within 1.3% or less, indicating that
INTRODUCTION All modern radiotherapy beam calibration protocols use water as the standard medium for dose determination.1,2 The primary reasons for this choice are multiple-fold: water is universally available, its physical properties are homogenous and uniform, and its beam absorption properties closely match that of soft tissues. It is, however, time consuming to setup a water tank, and measurement equipment, i.e., ion chambers, have to be waterproof. Over the last 2 decades, extensive efforts have been directed at developing solid phantom materials as substitute for natural water. This has lead to such commercial products as solid water (GAMMEX RMI, Middleton, WI); plastic water (Computerized Imaging References Systems, Inc., Norfolk, VA); and white water (PTW-Freiburgh, Freiburg, Germany). In particular, solid water has emerged as a leading candidate as a water-equivalent phantom material for both photon and electron beam output measurements.3– 6 In addition, solid water is also widely used as a phantom material for dose characterization of low-energy brachytherapy sources.7 Recently, a new solid phantom material, virtual water, was introduced by MED-TEC, Inc. (Orange City, IA.). The new material has the same chemical composition as Reprint requests to: Lizhong Liu, Ph.D., Department of Radiation Oncology, SUNY Upstate Medical University, 750 East Adams Street, Syracuse, NY 13210. E-mail: [email protected] This work was presented at the 2000 AAPM Annual Meeting in Chicago, IL 267
268
Medical Dosimetry
Table 1. Physical properties of solid water and virtual water phantom materials Nominal Density Measured Density (g/cm3) (g/cm3)
Material Solid Water 457 Virtual Water
1.015 1.03
Manufacturer
1.042 (⫾ 2%) GAMMEX RMI 1.070 (⫾ 1%) MED-TEC, Inc.
Volume 28, Number 4, 2003
Table 2. Water-equivalency correction factor for photon beams Photon Energy (MV) 6 15
correction factors should be applied in the calculation of beam outputs. MATERIALS AND METHODS The physical properties of the 2 solid phantom materials used in this work are listed in Table 1. Both the solid water model 457 and the virtual water are resins having the same chemical composition; however, the manufacturing processes differ, resulting in slightly different density and consistency. Both materials were designed for use in high-energy photon and electron beams. In our measurements, all phantom materials consisted of 30-cm-square slabs of various thickness, ranging from 0.2 cm, 0.5 cm, 1 cm, 2 cm, 5 cm, to 6 cm. The density of each slab was determined by measuring the weight and thickness using a weight scale and a micrometer, respectively. The measured density and its spread among the slabs are listed in Table 1. These values differ from the nominal density provided by the manufacturers by up to 4%. For each phantom material in our measurements, the 6-cm-thick slab had a cavity drilled out at the depth of 1 cm to accommodate an NE2571 (NE Technology Limited, Berkshire, England.) 0.6-cc Farmer-type ion chamber. The water tank used in this study was a Lucite box of 20-cm square by 30-cm high. Waterproofing of the NE2571 Farmer chamber was achieved using a 1-mmthick Lucite sleeve that fitted nicely on the ion chamber. A portable sealed water phantom with chamber cavity at 5-cm depth was also used for some of the measurements in photons. High-energy photon and electron beams were provided by a Varian 2100C linear accelerator (Varian Medical Systems, Palo Alto, CA). Depths of 5 cm and 10 cm were used for the measurements with 6- and 15-MV photon beams. For electron beams, depths at the maximum dose buildup were used for 5 electron beams at 6, 9, 12, 16, and 20 MeV. Measurements in both the solid materials and the water phantom were at the same linear depth, using the same ion chamber and electrometer system. A water-equivalency correction factor (WECF) is defined to quantify the degree of water equivalency for the solid phantom material. This factor is simply the ratio of the ion chamber charge reading in the water phantom to that in the solid material phantom at the same linear depth, i.e., WECF ⫽ (electrometer reading in water)/ (electrometer reading in solid material).
Depth (cm)
WECFsolid water
WECFvirtual water
5 10 5 10
0.986 0.988 0.993 0.992
0.992 0.993 0.998 0.998
RESULTS AND DISCUSSION The water-equivalency correction factors for the 6and 15-MV photon beams are shown in Table 2. No depth dependence between 5 and 10 cm was observed for both photon energies in both solid materials. The corrections for the 6-MV beam were slightly larger than the 15-MV beam for both materials. The newly developed virtual water material indeed showed improved water equivalency for photons with a maximum correction of 0.8%, as compared to 1.4% for the solid water. This was in general agreement with claims made by the manufacturer, but was slightly outside of the 0.5% range advertised. The water equivalency correction factors for the electron beams are shown in Table 3. Unlike the photon beams, no noticeable differences in the correction factors between the 2 solid materials were observed for the electron beams. Therefore, the newly developed virtual water had improved water equivalency in photon beams only. The correction factors for the 2 solid materials display a weak dependence on the electron energy, with a larger correction for the lower energies. The maximum correction was about 1.4% for both materials. It was also interesting to note that the correction factors in electron beams were always greater than unity, whereas those in photon beams were less than unity. The magnitudes of the corrections in both cases were, however, comparable, indicating that the chemical compositions of the 2 materials were optimized among all beam modalities and energies. Solid water model 457 was investigated in one previous study8 using several different measurement techniques at different linear (cm) or mass (g/cm2) depths, and with or without depth dose or depth ionization corrections in solid. Our measurement technique of using linear depth without depth dose or ionization corTable 3. Water-equivalency correction factor for electron beams Electron Energy (MeV)
Depth (cm)
WECFsolid water
WECFvirtual water
6 9 12 16 20
1.5 2.2 2.5 2.5 2.5
1.011 1.013 1.011 1.008 1.007
1.012 1.014 1.008 1.007 1.008
Water-equivalent phantom materials ● L. LIU et al.
rection corresponds to the techniques IV and I, respectively, for the photon and electron beams in Ref. 8. The authors of Ref. 8 reported that for photon beams from Cobalt-60 (60Co) to 24 MV, the ratio of dose in solid water materials over the dose in water varied from 1.009 to 0.989. This corresponded to a water-equivalency correction factor of 0.991 to 1.011, because the WECF defined in our study was essentially the inverse of the dose ratio. Excluding the lower and higher energies and the different linear accelerator models narrowed the spread of the correction factor to 0.992 to 1.002 for 6- to 15-MV photons. This was comparable with the range of 0.986 to 0.993 observed in this study for the same material (see Table 2). However, the 2 ranges appeared slightly shifted relative to each other by 0.6% to 0.9%, due perhaps to inconsistencies among phantom slabs from different batches. For electron beams of 6 to 15 MeV, a dose ratio of 0.985 to 0.99 (i.e., a WECF factor of 1.015 to 1.01) was reported in Ref. 8. This was in excellent agreement with our result of 1.013 to 1.007 for the same material (see Table 3). CONCLUSIONS Two commercially-available solid phantom materials were evaluated for their water equivalency in output calibration of photon and electron beams. The maximum differences in outputs measured in the 2 solid materials compared to water were found to be 1.4%, making both the solid water and the newly developed virtual water suitable substitutes for natural water in photon and elec-
269
tron beam output measurements. The virtual water was found to have improved water equivalency in photons as compared to solid water, however, the response in electrons were identical for both materials. The magnitudes of the correction, however, were not negligibly small, and the water equivalency correction factor should be included in the output calculation. These results indicate that any phantom material intended for beam output measurements should undergo a quality assurance test before being put into clinical use. REFERENCES 1. AAPM TG-21. A protocol for the determination of absorbed dose from high-energy photon and electron beams. Med. Phys. 10:741– 71; 1983. Erratum: A protocol for the determination of absorbed dose from high-energy photon and electron beams. Med. Phys. 11:213; 1984. 2. AAPM TG-51. AAPM’s TG-51 protocol for clinical reference dosimetry of high-energy photon and electron beams. Med. Phys. 26:1847–70; 1999. 3. Constantinou, C.; Attix, F.; Paliwal, B. A solid water phantom material for radiotherapy x-ray and x-ray beam calibrations. Med. Phys. 9:436 –41; 1982. 4. Ho, A.; Paliwal, B. Stopping-power and mass energy-absorption coefficient ratios for solid water. Med. Phys. 31:403–4; 1986. 5. Thomadsen, B.; Constantinou, C.; Ho, A. Evaluation of waterequivalent plastics as phantom material for electron beam dosimetry. Med. Phys. 22:291; 1995. 6. Prasad, S.C. Comparison of absorbed doses in water and solid water for electron beams. Med. Dosim. 17:205–6; 1992. 7. Meigooni, A.G.; Li, Z.; Mishra, V.; Williamson, J.F. A comparative study of dosimetric properties of plastic water and solid water in brachytherapy applications. Med. Phys. 21:1983–7; 1994. 8. Tello, V.M.; Tailor, R.C.; Hanson, W.F. How water equivalent are water-equivalent solid materials for output calibration of photon and electron beams? Med. Phys. 22:1177–89; 1995.