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Tin Shielding Thicknesses for Electrons
Medical Dosimetry, Vol. 23, No. 1, pp. 21–23, 1998 Copyright © 1998 American Association of Medical Dosimetrists Printed in the USA. All rights reserved 0958-3947/98 $19.00 1 .00
PII S0958-3947(97)00111-8
TIN SHIELDING THICKNESSES FOR ELECTRONS S. GURU PRASAD, K. PARTHASARADHI, W.H. AL-NAJJAR, and W.D. BLOOMER Division of Medical Physics, Northwestern University Medical School, Evanston Hospital, Evanston, IL 60201 Abstract—The transmission of 6, 9, 12, 16, and 20 MeV electrons from a linear accelerator through tin and lead is studied at 0.5 cm depth in polystyrene. These measurements are performed using a 10 3 10 cm2 cone and extend well into bremsstrahlung region. The results show certain advantages of using tin to shield sensitive organs from electron beams. Tin is non-toxic and creates less additional bremsstrahlung even though it must be thicker than the equivalent lead shield. © 1998 American Association of Medical Dosimetrists. Key Words: Linear accelerator, Tin, Lead, Bremsstrahlung.
INTRODUCTION
window caused by the weight of overlying metal sheets. Lead and tin foils are placed on the phantom surface, one on top of the other, and the charge measurements are performed until no significant reduction in charge is noticed. The consistency of the beam monitoring system was checked by performing charge measurements at the beginning and end of each run of transmission measurements. In addition, an average of three charge measurements is taken for each foil. All charge measurements are expressed as percentage of the open beam. The reproducibility of the charge measurements is within 0.5%. All foils are 30 cm 3 30 cm2 in size, and are obtained commercially. Since both tin and lead are soft and malleable materials, it is difficult to measure their thicknesses accurately over the entire area; however, the thickness for each foil is carefully measured at a number of points and the average value is used. The error in the thickness of lead and tin is approximately 4%. The comparison of tin and lead properties and their costs are presented in Table 1.
Lead masks are used to shape irregular fields in radiotherapy. Several authors1–5 have measured the transmission of electrons through lead and Lipowitz metal for electrons of different energies at various depths in a polystyrene phantom. It is observed that beyond the point of domination of bremsstrahlung, any addition of the shielding material is not of much use for further significant reduction of dose. Khan et al.6 analyzed the thickness of lead required to reduce the charge to the point of domination of bremsstrahlung and recommended, as a rule, that 1 mm thickness of lead should be used for every 2 MeV of electron energy. It is also recommended that for shielding the sensitive organs of patients, where the minimum thickness is desirable, actual measurements should be performed depending on the treatment area and depth. Farahani et al.7 demonstrated the use of Ag-Cu-polymer as an effective electron shielding material in spite of its higher physical thickness than Lipowitz metal. Tin, which is non-toxic and malleable, and can be obtained commercially, is considered as a shielding material for sensitive organs because it produces less bremsstrahlung than lead. Transmission measurements are performed through tin and lead for 6, 9, 12, 16, and 20 MeV electrons produced by Clinac 2100.
RESULTS AND DISCUSSION The percent transmission of charge is plotted against thickness for lead and tin as shown in Figs. 1 and 2. These figures show that as the energy increases the photon (bremsstrahlung) contribution increases in the tail portion. Rustgi and Rodgers, utilizing deflecting magnetic fields, determined that the photon component of the electron beams originates mostly within
METHODS AND MATERIALS All measurements are performed in a 25 3 25 cm2 polystyrene phantom using a Capintec PS-33 parallel plate chamber with 0.5 mg/cm2 window thickness and a Keithley electrometer. The parallel plate chamber is mounted in the phantom such that the entrance window of the detector is flush with the polystyrene surface. All measurements are performed for 10 cm 3 10 cm2 field size at 100 cm SSD. A polystyrene sheet of 0.5 cm is placed on the chamber to avoid the stress on the chamber
Table 1. Lead and tin properties and costs
Density Atomic number (Z) Toxicity Malleability Cost
Reprint requests to: Dr. S. Guru Prasad, Division of Medical Physics, Northwestern University Medical School, Evanston Hospital, 2650 Ridge Avenue, Evanston, IL 60201.
Lead
Tin
11.34 g/cm3 82 Highly toxic Soft and malleable
7.31 g/cm3 50 Non-toxic Soft and malleable About 30% higher than lead
Properties taken from Refs. 10 and 11. 21
22
Medical Dosimetry
Volume 23, Number 1, 1998
Fig. 1. Transmission curves for 6, 9, 12, 16, and 20 MeV electrons through tin for 10 cm 3 10 cm2 cone.
the collimator and scattering foil rather than within the water phantom.8 When water is used as an attenuating medium the photon components at the domination of bremsstrahlung for our electron beams are 0.4, 0.8, 1.5, 3.1, and 4.5%, respectively for 6, 9, 12, 16, and 20 MeV energies for a 10 3 10 cm2 cone. Any increase in photon component noticed with tin and lead over that of water is clearly originated from the metal foils but not from the collimator. For electrons, the bremsstrahlung production is less in tin than in lead;9 therefore, slightly additional reduction in transmission can be achieved with tin than with lead. In Fig. 3 semi-log plots of transmission vs. thickness for lead and tin for 20 MeV electrons are shown. The figure
Fig. 3. Semi-log transmission curves for lead and tin for 20 MeV electrons for 10 cm 3 10 cm2 cone. At the domination point of bremsstrahlung the corresponding percentage attenuations are 87% and 89% for lead and tin, respectively.
shows that beyond a certain thickness no further significant reduction in charge is noticed for the addition of foils. This point is defined as the point of domination of bremsstrahlung component. From this point onwards the plots are nearly linear, showing that the reduction is due to photon component only. The attenuation for tin and lead at these points are 89% and 87%, respectively. Attenuation for all electron energies for tin is measured at these respective points. The Table 2. Tin and lead thicknesses and percent charge reduction for electrons for 10 3 10 cm2 cone
Fig. 2. Transmission curves for 6, 9, 12, 16, and 20 MeV electrons through lead for 10 cm 3 10 cm2 cone.
Energy (MeV)
Mean incident energy (MeV)
Percent attenuation
6 9 12 16 20
4.5 7.6 10.8 14.3 18.4
98 97 95 92 89
Tin thickness (mm) 4.1 6.4 8.6 11.2 15.0
(3.0) (4.7) (6.3) (8.2) (11.0)
Lead thickness (mm) 2.7 4.5 6.0 7.9 11.0
(3.1) (5.1) (6.8) (9.0) (12.5)
The values in the brackets are the corresponding mass thicknesses (g/cm2) corrected to the last figure.
Tin shielding thicknesses for electrons ● S. G. PRASAD et al.
thickness of lead required to achieve the same percentage attenuation as tin is evaluated for all energies. These results are presented in Table 2. From the table a slight advantage of tin over lead in mass thickness, particularly for high-energy electrons, can be seen. From our investigations it is observed that tin can also be used for electron shielding for sensitive organs in spite of its higher physical thickness than lead, because of its malleability, non-toxicity, and less bremsstrahlung. The only constraint is that tin can only be used at temperatures above 13.2°C. Below this temperature tin undergoes certain structural and density changes.10 REFERENCES 1. Giarratano, J.C.; Duerkes, R.J.; Almond, P.R. Lead shielding thicknesses for the dose reduction of 7- to 28-MeV electrons. Med. Phys. 2:336 –337; 1975. 2. Khan, F.M.; Werner, B.L.; Deibel, F.C., Jr. Lead shielding for electrons. Med. Phys. 8:712–713; 1981. 3. Purdy, J.A.; Choi, M.C.; Feldman, A. Lipowitz metal shielding
23
thickness for dose reduction of 6 –20 MeV electrons. Med. Phys. 7:251–253; 1980. 4. Guru Prasad, S.; Parthasaradhi, K.; Arbetter, B.; Lee, Y.; Garces, R. Lead shielding thickness for the dose reduction of 6 MeV electrons for square fields. Med. Phys. 15:263–266; 1988. 5. Guru Prasad, S.; Parthasaradhi, K.; Lee, Y.; R. Garces. Lead shielding thickness for the dose reduction of 5 MeV electrons. Med. Phys. 16:807– 808; 1989. 6. Khan, F.H.; Doppke, K.P.; Hogstrom, K.R.; Kutcher, G.J.; Nath, R.V.; Prasad, S.C.; Purdy, J.A.; Rozenfeld, M.; Werner, B.L. Clinical electron beam dosimetry. Report of AAPM Radiation Therapy Committee Task Group 25. Med. Phys. 18:73– 121; 1991. 7. Farahani, M.; Eichmiller, F.C.; McLaughlin, W.L. New method for shielding electron beams used for head and neck cancer treatment. Med. Phys. 20:1237–1241; 1993. 8. Rustgi, S.N.; Rodgers, J.E. Analysis of the bremsstrahlung component in 6 –18 MeV electron beams. Med. Phys. 14:884 – 888; 1987. 9. Stopping powers for electrons and positrons. ICRU Report 37; 1984. 10. Merck Index. 11th ed. Rahway, NJ: Merck & Co., Inc.; 1989: 851,1488. 11. Goyer, R.A. Toxic effects of metals. In: Amdur, M.O.; Doull, J.; Klaassen, C.D., editors. The basic science of poisons. 4th ed. New York: Pergamon Press; 1991:623– 681.
PII S0958-3947(97)00111-8
TIN SHIELDING THICKNESSES FOR ELECTRONS S. GURU PRASAD, K. PARTHASARADHI, W.H. AL-NAJJAR, and W.D. BLOOMER Division of Medical Physics, Northwestern University Medical School, Evanston Hospital, Evanston, IL 60201 Abstract—The transmission of 6, 9, 12, 16, and 20 MeV electrons from a linear accelerator through tin and lead is studied at 0.5 cm depth in polystyrene. These measurements are performed using a 10 3 10 cm2 cone and extend well into bremsstrahlung region. The results show certain advantages of using tin to shield sensitive organs from electron beams. Tin is non-toxic and creates less additional bremsstrahlung even though it must be thicker than the equivalent lead shield. © 1998 American Association of Medical Dosimetrists. Key Words: Linear accelerator, Tin, Lead, Bremsstrahlung.
INTRODUCTION
window caused by the weight of overlying metal sheets. Lead and tin foils are placed on the phantom surface, one on top of the other, and the charge measurements are performed until no significant reduction in charge is noticed. The consistency of the beam monitoring system was checked by performing charge measurements at the beginning and end of each run of transmission measurements. In addition, an average of three charge measurements is taken for each foil. All charge measurements are expressed as percentage of the open beam. The reproducibility of the charge measurements is within 0.5%. All foils are 30 cm 3 30 cm2 in size, and are obtained commercially. Since both tin and lead are soft and malleable materials, it is difficult to measure their thicknesses accurately over the entire area; however, the thickness for each foil is carefully measured at a number of points and the average value is used. The error in the thickness of lead and tin is approximately 4%. The comparison of tin and lead properties and their costs are presented in Table 1.
Lead masks are used to shape irregular fields in radiotherapy. Several authors1–5 have measured the transmission of electrons through lead and Lipowitz metal for electrons of different energies at various depths in a polystyrene phantom. It is observed that beyond the point of domination of bremsstrahlung, any addition of the shielding material is not of much use for further significant reduction of dose. Khan et al.6 analyzed the thickness of lead required to reduce the charge to the point of domination of bremsstrahlung and recommended, as a rule, that 1 mm thickness of lead should be used for every 2 MeV of electron energy. It is also recommended that for shielding the sensitive organs of patients, where the minimum thickness is desirable, actual measurements should be performed depending on the treatment area and depth. Farahani et al.7 demonstrated the use of Ag-Cu-polymer as an effective electron shielding material in spite of its higher physical thickness than Lipowitz metal. Tin, which is non-toxic and malleable, and can be obtained commercially, is considered as a shielding material for sensitive organs because it produces less bremsstrahlung than lead. Transmission measurements are performed through tin and lead for 6, 9, 12, 16, and 20 MeV electrons produced by Clinac 2100.
RESULTS AND DISCUSSION The percent transmission of charge is plotted against thickness for lead and tin as shown in Figs. 1 and 2. These figures show that as the energy increases the photon (bremsstrahlung) contribution increases in the tail portion. Rustgi and Rodgers, utilizing deflecting magnetic fields, determined that the photon component of the electron beams originates mostly within
METHODS AND MATERIALS All measurements are performed in a 25 3 25 cm2 polystyrene phantom using a Capintec PS-33 parallel plate chamber with 0.5 mg/cm2 window thickness and a Keithley electrometer. The parallel plate chamber is mounted in the phantom such that the entrance window of the detector is flush with the polystyrene surface. All measurements are performed for 10 cm 3 10 cm2 field size at 100 cm SSD. A polystyrene sheet of 0.5 cm is placed on the chamber to avoid the stress on the chamber
Table 1. Lead and tin properties and costs
Density Atomic number (Z) Toxicity Malleability Cost
Reprint requests to: Dr. S. Guru Prasad, Division of Medical Physics, Northwestern University Medical School, Evanston Hospital, 2650 Ridge Avenue, Evanston, IL 60201.
Lead
Tin
11.34 g/cm3 82 Highly toxic Soft and malleable
7.31 g/cm3 50 Non-toxic Soft and malleable About 30% higher than lead
Properties taken from Refs. 10 and 11. 21
22
Medical Dosimetry
Volume 23, Number 1, 1998
Fig. 1. Transmission curves for 6, 9, 12, 16, and 20 MeV electrons through tin for 10 cm 3 10 cm2 cone.
the collimator and scattering foil rather than within the water phantom.8 When water is used as an attenuating medium the photon components at the domination of bremsstrahlung for our electron beams are 0.4, 0.8, 1.5, 3.1, and 4.5%, respectively for 6, 9, 12, 16, and 20 MeV energies for a 10 3 10 cm2 cone. Any increase in photon component noticed with tin and lead over that of water is clearly originated from the metal foils but not from the collimator. For electrons, the bremsstrahlung production is less in tin than in lead;9 therefore, slightly additional reduction in transmission can be achieved with tin than with lead. In Fig. 3 semi-log plots of transmission vs. thickness for lead and tin for 20 MeV electrons are shown. The figure
Fig. 3. Semi-log transmission curves for lead and tin for 20 MeV electrons for 10 cm 3 10 cm2 cone. At the domination point of bremsstrahlung the corresponding percentage attenuations are 87% and 89% for lead and tin, respectively.
shows that beyond a certain thickness no further significant reduction in charge is noticed for the addition of foils. This point is defined as the point of domination of bremsstrahlung component. From this point onwards the plots are nearly linear, showing that the reduction is due to photon component only. The attenuation for tin and lead at these points are 89% and 87%, respectively. Attenuation for all electron energies for tin is measured at these respective points. The Table 2. Tin and lead thicknesses and percent charge reduction for electrons for 10 3 10 cm2 cone
Fig. 2. Transmission curves for 6, 9, 12, 16, and 20 MeV electrons through lead for 10 cm 3 10 cm2 cone.
Energy (MeV)
Mean incident energy (MeV)
Percent attenuation
6 9 12 16 20
4.5 7.6 10.8 14.3 18.4
98 97 95 92 89
Tin thickness (mm) 4.1 6.4 8.6 11.2 15.0
(3.0) (4.7) (6.3) (8.2) (11.0)
Lead thickness (mm) 2.7 4.5 6.0 7.9 11.0
(3.1) (5.1) (6.8) (9.0) (12.5)
The values in the brackets are the corresponding mass thicknesses (g/cm2) corrected to the last figure.
Tin shielding thicknesses for electrons ● S. G. PRASAD et al.
thickness of lead required to achieve the same percentage attenuation as tin is evaluated for all energies. These results are presented in Table 2. From the table a slight advantage of tin over lead in mass thickness, particularly for high-energy electrons, can be seen. From our investigations it is observed that tin can also be used for electron shielding for sensitive organs in spite of its higher physical thickness than lead, because of its malleability, non-toxicity, and less bremsstrahlung. The only constraint is that tin can only be used at temperatures above 13.2°C. Below this temperature tin undergoes certain structural and density changes.10 REFERENCES 1. Giarratano, J.C.; Duerkes, R.J.; Almond, P.R. Lead shielding thicknesses for the dose reduction of 7- to 28-MeV electrons. Med. Phys. 2:336 –337; 1975. 2. Khan, F.M.; Werner, B.L.; Deibel, F.C., Jr. Lead shielding for electrons. Med. Phys. 8:712–713; 1981. 3. Purdy, J.A.; Choi, M.C.; Feldman, A. Lipowitz metal shielding
23
thickness for dose reduction of 6 –20 MeV electrons. Med. Phys. 7:251–253; 1980. 4. Guru Prasad, S.; Parthasaradhi, K.; Arbetter, B.; Lee, Y.; Garces, R. Lead shielding thickness for the dose reduction of 6 MeV electrons for square fields. Med. Phys. 15:263–266; 1988. 5. Guru Prasad, S.; Parthasaradhi, K.; Lee, Y.; R. Garces. Lead shielding thickness for the dose reduction of 5 MeV electrons. Med. Phys. 16:807– 808; 1989. 6. Khan, F.H.; Doppke, K.P.; Hogstrom, K.R.; Kutcher, G.J.; Nath, R.V.; Prasad, S.C.; Purdy, J.A.; Rozenfeld, M.; Werner, B.L. Clinical electron beam dosimetry. Report of AAPM Radiation Therapy Committee Task Group 25. Med. Phys. 18:73– 121; 1991. 7. Farahani, M.; Eichmiller, F.C.; McLaughlin, W.L. New method for shielding electron beams used for head and neck cancer treatment. Med. Phys. 20:1237–1241; 1993. 8. Rustgi, S.N.; Rodgers, J.E. Analysis of the bremsstrahlung component in 6 –18 MeV electron beams. Med. Phys. 14:884 – 888; 1987. 9. Stopping powers for electrons and positrons. ICRU Report 37; 1984. 10. Merck Index. 11th ed. Rahway, NJ: Merck & Co., Inc.; 1989: 851,1488. 11. Goyer, R.A. Toxic effects of metals. In: Amdur, M.O.; Doull, J.; Klaassen, C.D., editors. The basic science of poisons. 4th ed. New York: Pergamon Press; 1991:623– 681.