- Email: [email protected]
Measurement of photoneutron doses in and out of high-energy X-ray beam of a SATURNE-20 medical linear accelerator by ECE polycarbonate detectors
Radiation Measurements
PERGAMON
Radiation Measurements31 (1999) 479--482
M E A S U R E M E N T O F P H O T O N E U T R O N D O S E S IN A N D O U T OF HIGH-ENERGY X-RAY BEAM OF A SATURNE-20 MEDICAL LINEAR ACCELERATOR BY ECE POLYCARBONATE
DETECTORS
M. SOHRABI AND A. MOSTOFIZADEH Nat. Radiat. Protection Department & Center for Research on Natural Radiation, Atomic Energy Organization of Iran, P. O. Box 14155-4494, Tehran, Islamic Republic of Iran
ABSTRACT Photoneutron contaminations in and out of high energy X-ray beams of the medical linear accelerator SATURNE 20 (CGR) of the Radiotherapy Department of Omeed Hospital in Isfahan, Iran, have been determined using 250 ~rn polycarbonate (PC) dosimeters, m slrips or in sheets, processed by electrochemical etching (ECE) using specially designed ECE chambers to etch larger sheets. A two dimensional or topographical distribution of neutron contamination was also determined in a full size beam. The neutron dose equivalents (Hn) in the beam of 18 MV X-rays at 80 cm FSD were determined to be linear functions of Xray dose equivalents (Hx) up to 1400 cSv. The distribution of the Hn at different X-ray doses showed bell-shape profiles with maxima at the isocenter. The ratios of dose equivalents of neutrons to those of X-rays increased as the field size increased having values of 0.22%, 0.28%, 0.31% and 0.37% for field sizes of 10xl0, 20x20, 30x30o and 40x40 cm2 respectively. Although such neutron dose equivalents can be corrected for patient treatment, it can cause radiation protection problems for workers where the design of the facility is not well planned.
KEYWORDS Medical accelerator; radiotherapy; high energy X-rays; photoneutrons; polycarbonate dosimeters; electrochemical etching.
neutron
dose; measurements;
INTRODUCTION Photoneutrons exist in and out of high energy X-ray beams of medical linear accelerators whenever the photon energy exceeds the energy thresholds of the (% n) and (e, e'n) type reactions in the target, collimator and beam flattening filter materials. Many studies have been carded out in the high energy beams on protection of patients and workers (Sohrabi, 1975; McGinley et al., 1976; Sohrabi and Morgan, 1979; NCRP, 1984), on calculation of cross sections (Dietrich and Berman, 1988), on such neutron sources (McCall and Swanson, 1979; Mao et al., 1996), on activation of materials and patient (Powell et al., 1987), etc. No such studies have yet been performed at the medical linear accelerator SATURNE-20 (CGR) of the Radiotherapy D e p a ~ n e n t of the Omeed Hospital in Isfahan, Iran. Therefore, a detailed survey of neutron contaminations in and around of this accelerator was performed for the protection of workers and patients using PC dosimeters processed by ECE (Sohrabi, 1974; Sohrabi and Morgan, 1978), and Neutriran Albedo Neutron Personnel Dosimeter (NANPD) (Sohrabi, 1979; Sohrabi and Katouzi, 1991). Neutron dose equivalents (I-In) have been investigated as a function of distance from the isocenter, both in and out of the beams of different field sizes, around the head, at different points inside the treatment room, inside the maze, on the treatment room door, inside the control room and on the personnel, as well as in the beam to obtain neutron topographical distribution in a full field size. It is the purpose of this paper to present some of the data obtained by the PC dosimeters in and out of the photon beam at different field sizes. The results of other studies will be reported elsewhere.
1350-4487/99/$- see front matter © 1999 ElsevierScience Ltd. All rights reserved. PII:S 1350-4487(99)00203-6
M. Sohrabi, A. Mostofizadeh / Radiation Measurements 31 11999) 479-482
480
EXPERIMENTS AND METHODS A 20 MV accelerator SATURNE-20, Model 1973, from CGR of France was used. In our previous neutron contamination studies of a Varian Clinac-18, an Allis-Chalmers 25 MeV betatron and a Brown Bovery 45 MeV betatron at the Emory University Hospital Center in Atlanta, Ga., PC dosimeters of 250 ~tm thickness proved to have good characteristics (Sohrabi and Morgan, 1979; McGinley and Sohrabi, 1979). The PC dosimeter was proved to be highly insensitive to such high energy X-ray beams; thus most ideal for such studies. The PC dosimeters were used as strips laid on the patient bench for dose distribution in and out of the beam, or as a sheet larger than the field size to obtain topological dose distribution. They were processed by the ECE method, either using the ECE chamber system recently developed (Sohrabi, 1993), or a chamber able to etch a strip of dosimeter with an effective area of 5 x 25 cm 2, specially designed for this purpose. The PC dosimeters were processed in a PMW solution (15g KOH + 40g CH3OH + 45g H20) at 25 °C applying a field strengt h of 32 kV cm -~ at 2 kHz for 2.5 h. By using PMW for ECE processing, the background track density is about three times lower than that with the PEW etchant. After the ECE, one can easily observe, by the unaided eyes, any changes in the track density in the strip which is important for checking the beam uniformity. The dosimeters were calibrated to fission neutrons from a 10 mg 252Cf neutron source processed by ECE using KOH, PMW and PEW etchants. RESULTS AND DISCUSSION Using the PC dosimeters, the distribution of the photoneutron dose equivalents (I-In) as functions of the distance from the isocenter were determined for five different values of photon dose equivalents (Hx) up to 1428 cSv, for a 20 x 20 cm 2 field, as shown in Fig. 1. As it can be seen, the photoneutron contamination in the beam have bell-shape profiles with maxima at the isocenter and lowest values at the beam edges. The response of the Hn at the isocenter as function of Hx is linear, as shown in Fig. 2, with a function:
(1)
Hn(Sv) = 2.8 x 10-3 Hx(Sv). A similar experiment for a 40 x 40 cm: field led also to a linear response with a function: Hn(Sv) = 3.1 x 10-3 Hx(Sv).
(2)
As it can be observed, the neutron contamination of the 40 x 40 cm 2 field is 1.1 times higher than that of a 20 x 20 cm 2 field. In fact as the field size increased, the neutron contamination also increased. 50 IECE" with PMW : 800 V,2kHz,2.5hr,25"C
A 1428 cSv 11000 MU) o 1143 cSv (800
.,40.3
40 E e-, ¢1
MU)
• 857 • 571
cSv 1600 MU) cSv 1400 MU)
-
csv(200
286
MU)
30
.>_ -i &
20 0
0
10 Z 0
- 40
t
-30
I
I
I
I
-20
-10
0
10
Distance ltom lsocenter (Cm)
I ,,
20
!
30
40
481
M. SohrabL A. Mostoflzadeh / Radiation Measurements 31 (1999) 479-482 hi}
60
1 ECE w i t h P H W : 800 V, 2 k H z , 2 . S h t , 2 5 " C -
t~j.
hO
S
~o ,~
0 5 2 t,
o t,0,t.0 c m 2 ~ 30,30 cm 2
A/,z £ ~ , . . \
, 10,10 t i m 2
50 /¢r ~O
qr}
311
28
z
10
10
z,OO
200
fl00
800
1000
12OU
lt, U(I
0 ..... -~0
i ..... -30
L - - __ I -20 -tO
I 10
,
I. . . . 20
J. . . . 30
40
[,)istancc from Iso¢entcr ( C m )
I ' h . t o n I),,~c i tltll~ aicll11111%~ )
Fig, 2. Hn versus Hx for a 20 x 20 cm field size,
I O
2
Fig. 3. Hn versus distance from isocenter for different field sizes.
Figure 3 shows the distribution of Hn versus distance from the isocenter for 10xl0, 20x20, 30x30, and 40x40 cm 2 field sizes at 80 cm FSD, for Hx of 1428 cSv. Again different bell-shape profiles were obtained for different field sizes. The neutron contamination increased as the field size increased. This is due to having less shielding effect for neutrons as the field size increases. The ratios of H, to Hx increased as the field size increased having values of 0.22%, 0.28%, 0.31% and 0.37% for field sizes of 10xl0, 20x20, 30x30, and 40x40 cm 2 respectively. Such values are comparable to the results obtained in the previous studies (Sohrabi and Morgan, 1979). It was of interest also to determine the two dimensional distribution of the photoneutron contamination of the beam for two purposes: first to demonstrate the ability of PC dosimeters and the ECE process for large size beam studies
404
ECE with PMW: 800V,2kHz,2.Shr.25°C
E 3O
~d 20 10
,Z
Fig. 4. Two dimensional Hn distribution in a 20x20
cm 2
field.
482
M. Sohrabi,A. Mostofizadeh/Radiation Measurements31 (1999)479-482
and second to determine the topological distribution of Fin in a two dimensional field. Figure 4 shows a two dimensional distribution of Hn for a photon dose equivalent of 1428 cSv (1000 MU), using a PC sheet of 60 x 60 cm2 for a field size of 20 x 20 cm2. As it can be seen, the peak is right at the isocenter. CONCLUSION The reponse of H, versus Hx, as expected, was linear for the beam of 18 MV X-rays up to 1400 cSv. The distribution of the Hn in the beam at different values of Hx showed bell-shape profiles with maxima at the isocenter. The ratios Ha to Hx increased as the field size increased having values of 0.22%, 0.28%, 0.31% and 0.37% for field sizes of 10xl0, 20x20, 30x30, and 40x40 cm2 respectively, at 80 cm FSD. The PC dosimeters processed by ECE proved to be versatile for studies of one or two dimensional distributions of photoneutron contamination in high energy X-ray beams with the main advantages of insensitivity to such photons as well as requiring only one single dosimeter and one single exposure.
Acknowledgment- Sincere co-operations of the medical physicists of the Omeed Hospital Dr. Shokram, Dr. Gharaati, and Mr. Sh. Monadi for beam exposures are highlyappreciated.
REFERENCES Dietrich, S. S. and Berman, B. L. (1988) Atlas of photoneutron cross sections obtained with monoenergetic photons. Atomic Data and Nuclear Data Tables 38, 199-338. McCall, R. C. and Swanson,, W.P. (1979) Neutron sources and their characteristics. National Bureau of Standards; NBS Special Publication No. 554, US Government Printing Office, Washington D.C., pp. 75-86. Mao, X. S., Kase, K. R., Liu, J. C., Nelson, W. R., Kleck J. H. and Johnson, S. (1997)., Neutron sources in the Varian Clinac 2100C/2300C medical accelerator calculated by the EGS4 Code, Health Phys. 72, 524-529. McGinley, P. H., Wood, M., Sohrabi, M., and Miles, M. (1976) Dose levels due to neutrons in the vicinity of high energy medical accelerators. In: Proc. 9th Midyear Topical Symp., Health Physics Society, Denver, Colorado, pp. 468-474 McGinley, P. H, and Sohrabi, M. (1979) Neutron Contamination in the Primary Beam. National Bureau of Standards; NBS Special Publication No. 554, US Government Printing Office, Washington D.C., pp. 99-107. National Council on Radiation Protection and Measurements (1984) Neutron contamination from medical electron accelerators. Washington D.C., NCRP, NCRP Report No. 79. Powell, N. L., Newing, A., Bullen, M. A., Sims, C. and Leaton, S. F. (1987) Radiation safety survey on a Clinac-20 linear accelerator. Phys. Med. Biol. 32, 707-718. Sohrabi, M. (1974) Electrochemical etching amplification of recoil particle tracks in polymers and its application in fast neutron personnel dosimetry". Health Physics 27, 598-600. Sohrabi, M. (1979) A new dual response albedo neutron personnel dosimeter. Nucl. Instrum. Meths. 165, 135-138. Sohrabi, M. (1993) A new triplet electrochemical (TECE) method, Radiat. Prot. Dos. 48, 279-283. Sohrabi, M. and Katouzi, M. (1991) Design characteristics of a three-component AEOI Neutriran albedo neutron personnel dosimeter. Nucl. Tracks Radiat. Meas. 19, 537-540. Sohrabi, M. and Morgan, K. Z. (1978) A new polycarbonate fast neutron personnel dosimeter. Amer. Ind. Hyg. J. 39, 438-447. Sohrabi, M. and Morgan, K. Z. (1979) Neutron dosimetry in high energy X-ray beams of medical accelerators. Phys. Ailed. Biol. 24, 756-766.
PERGAMON
Radiation Measurements31 (1999) 479--482
M E A S U R E M E N T O F P H O T O N E U T R O N D O S E S IN A N D O U T OF HIGH-ENERGY X-RAY BEAM OF A SATURNE-20 MEDICAL LINEAR ACCELERATOR BY ECE POLYCARBONATE
DETECTORS
M. SOHRABI AND A. MOSTOFIZADEH Nat. Radiat. Protection Department & Center for Research on Natural Radiation, Atomic Energy Organization of Iran, P. O. Box 14155-4494, Tehran, Islamic Republic of Iran
ABSTRACT Photoneutron contaminations in and out of high energy X-ray beams of the medical linear accelerator SATURNE 20 (CGR) of the Radiotherapy Department of Omeed Hospital in Isfahan, Iran, have been determined using 250 ~rn polycarbonate (PC) dosimeters, m slrips or in sheets, processed by electrochemical etching (ECE) using specially designed ECE chambers to etch larger sheets. A two dimensional or topographical distribution of neutron contamination was also determined in a full size beam. The neutron dose equivalents (Hn) in the beam of 18 MV X-rays at 80 cm FSD were determined to be linear functions of Xray dose equivalents (Hx) up to 1400 cSv. The distribution of the Hn at different X-ray doses showed bell-shape profiles with maxima at the isocenter. The ratios of dose equivalents of neutrons to those of X-rays increased as the field size increased having values of 0.22%, 0.28%, 0.31% and 0.37% for field sizes of 10xl0, 20x20, 30x30o and 40x40 cm2 respectively. Although such neutron dose equivalents can be corrected for patient treatment, it can cause radiation protection problems for workers where the design of the facility is not well planned.
KEYWORDS Medical accelerator; radiotherapy; high energy X-rays; photoneutrons; polycarbonate dosimeters; electrochemical etching.
neutron
dose; measurements;
INTRODUCTION Photoneutrons exist in and out of high energy X-ray beams of medical linear accelerators whenever the photon energy exceeds the energy thresholds of the (% n) and (e, e'n) type reactions in the target, collimator and beam flattening filter materials. Many studies have been carded out in the high energy beams on protection of patients and workers (Sohrabi, 1975; McGinley et al., 1976; Sohrabi and Morgan, 1979; NCRP, 1984), on calculation of cross sections (Dietrich and Berman, 1988), on such neutron sources (McCall and Swanson, 1979; Mao et al., 1996), on activation of materials and patient (Powell et al., 1987), etc. No such studies have yet been performed at the medical linear accelerator SATURNE-20 (CGR) of the Radiotherapy D e p a ~ n e n t of the Omeed Hospital in Isfahan, Iran. Therefore, a detailed survey of neutron contaminations in and around of this accelerator was performed for the protection of workers and patients using PC dosimeters processed by ECE (Sohrabi, 1974; Sohrabi and Morgan, 1978), and Neutriran Albedo Neutron Personnel Dosimeter (NANPD) (Sohrabi, 1979; Sohrabi and Katouzi, 1991). Neutron dose equivalents (I-In) have been investigated as a function of distance from the isocenter, both in and out of the beams of different field sizes, around the head, at different points inside the treatment room, inside the maze, on the treatment room door, inside the control room and on the personnel, as well as in the beam to obtain neutron topographical distribution in a full field size. It is the purpose of this paper to present some of the data obtained by the PC dosimeters in and out of the photon beam at different field sizes. The results of other studies will be reported elsewhere.
1350-4487/99/$- see front matter © 1999 ElsevierScience Ltd. All rights reserved. PII:S 1350-4487(99)00203-6
M. Sohrabi, A. Mostofizadeh / Radiation Measurements 31 11999) 479-482
480
EXPERIMENTS AND METHODS A 20 MV accelerator SATURNE-20, Model 1973, from CGR of France was used. In our previous neutron contamination studies of a Varian Clinac-18, an Allis-Chalmers 25 MeV betatron and a Brown Bovery 45 MeV betatron at the Emory University Hospital Center in Atlanta, Ga., PC dosimeters of 250 ~tm thickness proved to have good characteristics (Sohrabi and Morgan, 1979; McGinley and Sohrabi, 1979). The PC dosimeter was proved to be highly insensitive to such high energy X-ray beams; thus most ideal for such studies. The PC dosimeters were used as strips laid on the patient bench for dose distribution in and out of the beam, or as a sheet larger than the field size to obtain topological dose distribution. They were processed by the ECE method, either using the ECE chamber system recently developed (Sohrabi, 1993), or a chamber able to etch a strip of dosimeter with an effective area of 5 x 25 cm 2, specially designed for this purpose. The PC dosimeters were processed in a PMW solution (15g KOH + 40g CH3OH + 45g H20) at 25 °C applying a field strengt h of 32 kV cm -~ at 2 kHz for 2.5 h. By using PMW for ECE processing, the background track density is about three times lower than that with the PEW etchant. After the ECE, one can easily observe, by the unaided eyes, any changes in the track density in the strip which is important for checking the beam uniformity. The dosimeters were calibrated to fission neutrons from a 10 mg 252Cf neutron source processed by ECE using KOH, PMW and PEW etchants. RESULTS AND DISCUSSION Using the PC dosimeters, the distribution of the photoneutron dose equivalents (I-In) as functions of the distance from the isocenter were determined for five different values of photon dose equivalents (Hx) up to 1428 cSv, for a 20 x 20 cm 2 field, as shown in Fig. 1. As it can be seen, the photoneutron contamination in the beam have bell-shape profiles with maxima at the isocenter and lowest values at the beam edges. The response of the Hn at the isocenter as function of Hx is linear, as shown in Fig. 2, with a function:
(1)
Hn(Sv) = 2.8 x 10-3 Hx(Sv). A similar experiment for a 40 x 40 cm: field led also to a linear response with a function: Hn(Sv) = 3.1 x 10-3 Hx(Sv).
(2)
As it can be observed, the neutron contamination of the 40 x 40 cm 2 field is 1.1 times higher than that of a 20 x 20 cm 2 field. In fact as the field size increased, the neutron contamination also increased. 50 IECE" with PMW : 800 V,2kHz,2.5hr,25"C
A 1428 cSv 11000 MU) o 1143 cSv (800
.,40.3
40 E e-, ¢1
MU)
• 857 • 571
cSv 1600 MU) cSv 1400 MU)
-
csv(200
286
MU)
30
.>_ -i &
20 0
0
10 Z 0
- 40
t
-30
I
I
I
I
-20
-10
0
10
Distance ltom lsocenter (Cm)
I ,,
20
!
30
40
481
M. SohrabL A. Mostoflzadeh / Radiation Measurements 31 (1999) 479-482 hi}
60
1 ECE w i t h P H W : 800 V, 2 k H z , 2 . S h t , 2 5 " C -
t~j.
hO
S
~o ,~
0 5 2 t,
o t,0,t.0 c m 2 ~ 30,30 cm 2
A/,z £ ~ , . . \
, 10,10 t i m 2
50 /¢r ~O
qr}
311
28
z
10
10
z,OO
200
fl00
800
1000
12OU
lt, U(I
0 ..... -~0
i ..... -30
L - - __ I -20 -tO
I 10
,
I. . . . 20
J. . . . 30
40
[,)istancc from Iso¢entcr ( C m )
I ' h . t o n I),,~c i tltll~ aicll11111%~ )
Fig, 2. Hn versus Hx for a 20 x 20 cm field size,
I O
2
Fig. 3. Hn versus distance from isocenter for different field sizes.
Figure 3 shows the distribution of Hn versus distance from the isocenter for 10xl0, 20x20, 30x30, and 40x40 cm 2 field sizes at 80 cm FSD, for Hx of 1428 cSv. Again different bell-shape profiles were obtained for different field sizes. The neutron contamination increased as the field size increased. This is due to having less shielding effect for neutrons as the field size increases. The ratios of H, to Hx increased as the field size increased having values of 0.22%, 0.28%, 0.31% and 0.37% for field sizes of 10xl0, 20x20, 30x30, and 40x40 cm 2 respectively. Such values are comparable to the results obtained in the previous studies (Sohrabi and Morgan, 1979). It was of interest also to determine the two dimensional distribution of the photoneutron contamination of the beam for two purposes: first to demonstrate the ability of PC dosimeters and the ECE process for large size beam studies
404
ECE with PMW: 800V,2kHz,2.Shr.25°C
E 3O
~d 20 10
,Z
Fig. 4. Two dimensional Hn distribution in a 20x20
cm 2
field.
482
M. Sohrabi,A. Mostofizadeh/Radiation Measurements31 (1999)479-482
and second to determine the topological distribution of Fin in a two dimensional field. Figure 4 shows a two dimensional distribution of Hn for a photon dose equivalent of 1428 cSv (1000 MU), using a PC sheet of 60 x 60 cm2 for a field size of 20 x 20 cm2. As it can be seen, the peak is right at the isocenter. CONCLUSION The reponse of H, versus Hx, as expected, was linear for the beam of 18 MV X-rays up to 1400 cSv. The distribution of the Hn in the beam at different values of Hx showed bell-shape profiles with maxima at the isocenter. The ratios Ha to Hx increased as the field size increased having values of 0.22%, 0.28%, 0.31% and 0.37% for field sizes of 10xl0, 20x20, 30x30, and 40x40 cm2 respectively, at 80 cm FSD. The PC dosimeters processed by ECE proved to be versatile for studies of one or two dimensional distributions of photoneutron contamination in high energy X-ray beams with the main advantages of insensitivity to such photons as well as requiring only one single dosimeter and one single exposure.
Acknowledgment- Sincere co-operations of the medical physicists of the Omeed Hospital Dr. Shokram, Dr. Gharaati, and Mr. Sh. Monadi for beam exposures are highlyappreciated.
REFERENCES Dietrich, S. S. and Berman, B. L. (1988) Atlas of photoneutron cross sections obtained with monoenergetic photons. Atomic Data and Nuclear Data Tables 38, 199-338. McCall, R. C. and Swanson,, W.P. (1979) Neutron sources and their characteristics. National Bureau of Standards; NBS Special Publication No. 554, US Government Printing Office, Washington D.C., pp. 75-86. Mao, X. S., Kase, K. R., Liu, J. C., Nelson, W. R., Kleck J. H. and Johnson, S. (1997)., Neutron sources in the Varian Clinac 2100C/2300C medical accelerator calculated by the EGS4 Code, Health Phys. 72, 524-529. McGinley, P. H., Wood, M., Sohrabi, M., and Miles, M. (1976) Dose levels due to neutrons in the vicinity of high energy medical accelerators. In: Proc. 9th Midyear Topical Symp., Health Physics Society, Denver, Colorado, pp. 468-474 McGinley, P. H, and Sohrabi, M. (1979) Neutron Contamination in the Primary Beam. National Bureau of Standards; NBS Special Publication No. 554, US Government Printing Office, Washington D.C., pp. 99-107. National Council on Radiation Protection and Measurements (1984) Neutron contamination from medical electron accelerators. Washington D.C., NCRP, NCRP Report No. 79. Powell, N. L., Newing, A., Bullen, M. A., Sims, C. and Leaton, S. F. (1987) Radiation safety survey on a Clinac-20 linear accelerator. Phys. Med. Biol. 32, 707-718. Sohrabi, M. (1974) Electrochemical etching amplification of recoil particle tracks in polymers and its application in fast neutron personnel dosimetry". Health Physics 27, 598-600. Sohrabi, M. (1979) A new dual response albedo neutron personnel dosimeter. Nucl. Instrum. Meths. 165, 135-138. Sohrabi, M. (1993) A new triplet electrochemical (TECE) method, Radiat. Prot. Dos. 48, 279-283. Sohrabi, M. and Katouzi, M. (1991) Design characteristics of a three-component AEOI Neutriran albedo neutron personnel dosimeter. Nucl. Tracks Radiat. Meas. 19, 537-540. Sohrabi, M. and Morgan, K. Z. (1978) A new polycarbonate fast neutron personnel dosimeter. Amer. Ind. Hyg. J. 39, 438-447. Sohrabi, M. and Morgan, K. Z. (1979) Neutron dosimetry in high energy X-ray beams of medical accelerators. Phys. Ailed. Biol. 24, 756-766.