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16Mitch-BrachyPrimaryStandards.pdf
1. Primary Standards
f B h h S
for Brachytherapy Sources
Michael G. Mitch, Ph.D.
Christopher G. Soares, Ph.D.
Ph i L b t N ti l I tit t f St d d d T h l (NIST)
Physics Laboratory, National Institute of Standards and Technology (NIST)
2. Photon-emitting sources
g
L 50 k V Hi h
Low-energy < 50 keV < High-energy
LDR HDR LDR HDR
SK SK SK SK, Dw
WAFAC FAC Cavity Cavity, Calorimeter
3. Sources are Calibrated in Terms of Air-Kerma Strength (U)
d
Source Air Volume
Source Air Volume
Vacuum
SK = K (d) d 2
1 U = 1 Gy m2 h-1
.
• Air-kerma strength is the product of the air-kerma rate, in vacuo and due
1 U 1 Gy m h
to photons of energy greater than , at distance d and the square of this
distance.
Ai k b d b l l i h f i h b
• Air kerma can be measured absolutely with a free-air chamber
4. Low-energy, LDR seeds 20 keV < E < 35 keV
Ag wire, end welds
Ag wire, end welds
Ag spheres, end welds
Resin spheres, Au-Cu markers
W wire, double wall
5. • Since 1999, NIST has calibrated over 900 sources
• 40 designs from 18 manufacturers
3 di lid 125I 103Pd 131C
• 3 radionuclides: 125I, 103Pd, 131Cs
6. Primary Standard for Low-Energy X Rays (20 kV to 100 kV):
Ritz Free-Air Chamber (125I seeds, 1985)
V
Source
(4 to 6 125I seeds)
Electrometer
(4 to 6 125I seeds)
Collecting Volume = 440 cm3
Active Volume = 5.5 cm3
Collecting/Active Volumes = 80
8. 160 mm Al Center
Wide-Angle Free-Air Chamber (WAFAC)
Al Filt
Electrode
Al Filter
W
Electrometer
Rotating
Source
Aperture
V/ 2
V = - 1674 V
Aluminized Mylar
Electrodes
9. 43 mm Al Center
Wide-Angle Free-Air Chamber (WAFAC)
Al Filt
Electrode
Al Filter
W
Electrometer
Rotating
Source
Aperture
V/ 2
V = - 450 V
Aluminized Mylar
Electrodes
10. Air-Kerma Strength from WAFAC Measurements
j
j
i
i
det
dr
eff
air
air
K Q
K
K
Q
K
M
K
K
V
d
e
W
d
Q
K
S )
(
)
,
(
)
(
)
(
2
2
125I
Net current, sI 0.06
Value Type A (%) Type B (%)
Net current, s 0.06
)
,
(
det Q
K
M
I
e
W / 33.97 J / C - 0.15
Air density, ρair 1.196 mg / cm3 - 0.03
Aperture distance, d - 0.24
Effective chamber volume, Veff 0.11 0.01
Decay correction, K1 T1/2 = 59.43 d - 0.02
Recombination < 1 004 0 05
)
(K
K
Recombination, < 1.004 - 0.05
Attenuation in filter, K3(Q) 1.0295 - 0.61
Air attenuation in WAFAC, K4(Q) 1.0042 - 0.08
Source-aperture attenuation, K5(Q) 1.0125 - 0.24
Inverse-square correction, K6 1.0089 - 0.01
Humidity, K7(Q) 0.9982 - 0.07
)
(K
Kdr
Humidity, K7(Q) 0.9982 0.07
In-chamber photon scatter, K8(Q) 0.9966 - 0.07
Source-holder scatter, K9 0.9985 - 0.05
Electron loss, K10 1.0 - 0.05
Aperture penetration, K11(Q) 0.9999 - 0.02
External photon scatter, K12(Q) 1.0 - 0.17
Combined standard uncertainty, uc (s2 + 0.7622)1/2
Expanded uncertainty, V 2uc
11. Original and Automated WAFACs
Automated
HPGe
Spectrometer
Al filter
wheel
Original
WAFAC
WAFAC
seed
WAFAC
seed
12. Reference air kerma of low-energy photon-emitting LDR sources
• GROVEX (Grossvolumen Extrapolationskammer) – PTB
• VAFAC (Variable-Aperture Free-Air Chamber) – University of Wisconsin
• 3 L thin-walled cavity chamber radionuclide calibrator – NPL
• Torus free-air chamber – LNHB
13. Characterization Measurements Following WAFAC Calibration:
1. rotational anisotropy (WAFAC)
2 x-ray spectrometry on transverse axis of seed
2. x ray spectrometry on transverse axis of seed
3. well-ionization chamber response relative to WAFAC (I / SK)
4. exposure of radiochromic film (contact geometry)
5 l t t ( )
5. angular x-ray spectrometry (A)
14. Quality Assurance for WAFAC Measurements:
1. 241Am check source
2 x-ray spectrometry on transverse axis of seed
2. x ray spectrometry on transverse axis of seed
3. well-ionization chamber response relative to WAFAC (I / SK)
15. 1.06
263 seeds, 2003-2005:
Range = (7 ± 5) %
Rotational Anisotropy (WAFAC)
1.00
1.02
1.04
ve
Current
8 %
0 94
0.96
0.98
Relativ
z
0.94
-20 20 60 100 140 180 220 260 300 340 380
Rotation Angle (degrees)
1.06
x
y
1.02
1.04
Current
2 %
x
= 0, 45, 90…360o
0.96
0.98
1.00
Relative
C
2 %
0.94
-20 20 60 100 140 180 220 260 300 340 380
Rotation Angle (degrees)
16. X-ray spectrometry of 103Pd and 131Cs seeds
100000
120000
Rh K
103Pd: EC, T1/2 = 16.99 d
60000
80000
100000
C
o
u
n
ts
Rh K
Rh K 20.1 keV
Rh K 22.7 keV, 23.2 keV
20000
40000
C
Rh K
0
5 10 15 20 25 30 35 40
Energy (keV)
X K
150000
200000
nts
131Cs: EC, T1/2 = 9.69 d
Xe K
50000
100000
Coun
Xe K 29.4 keV, 29.8 keV
Xe K 33.6 keV, 34.4 keV
Xe K
0
5 10 15 20 25 30 35 40
Energy (keV)
17. X-ray spectrometry of 125I seeds
100000
120000
Te K
125I: EC, T1/2 = 59.43 d
T K 27 2 k V 27 5 k V
60000
80000
Counts
Te K 27.2 keV, 27.5 keV
Te K 31.0 keV, 31.7 keV
125I 35.5 keV 20000
40000
C
Te K
125I
100000
0
5 10 15 20 25 30 35 40
Energy (keV)
T K
Ag K 22 1 keV 60000
80000
100000
nts
125I (Ag)
Te K
Ag K 22.1 keV
Ag K 24.9 keV, 25.4 keV
Te K 27.2 keV, 27.5 keV
Te K 31.0 keV, 31.7 keV 20000
40000
Coun
Te K
125I
Ag K
Ag K
125I 35.5 keV
0
5 10 15 20 25 30 35 40
Energy (keV)
Ag K
18. Ionization Chambers
NIST chambers
Commercially available chambers
Spherical Aluminum
Capintec
CRC-127R1
Standard Imaging
HDR-1000 Plus1
Seltzer-Mitch
1Certain commercial equipment, instruments, and materials are identified in this work in order
to specify adequately the experimental procedure. Such identification does not imply
recommendation nor endorsement by the National Institute of Standards and Technology,
nor does it imply that the material or equipment identified is necessarily the best available
for these purposes.
19. Well Chamber Measurement Geometry
• Seed placed in end of catheter
• Centering jig for catheter
• Optimum vertical positioning
• Orientational effects:
1) up/down
1) up/down
2) “points-of-compass”
20. Well Chamber Response Coefficients for 22 Seed Models
5.3
5.8
I (pA)
S (U)
4.3
4.8
no Ag
131Cs
SK (U)
3.8
I
/
S
K
(pA
/
U)
12
Ag wires
no Ag
2.8
3.3
103Pd
125I
Ag spheres
Ag spheres
1.8
2.3
21. Control Chart, I / SK, 125I seed “D”
4 45
4.50
4.35
4.40
4.45
4.25
4.30
I
/
S
K
(pA
/
U)
4.10
4.15
4.20
I
4.05
Apr05 Aug05 Jul06 Feb07 Nov07
22. I / SK vs. Ag K / , 125I seed “D”
4 45
4.50
4 30
4.35
4.40
4.45
/
U)
4.20
4.25
4.30
I
/
S
K
(pA
/
4.05
4.10
4.15
2 5 2 7 2 9 3 1 3 3 3 5 3 7 3 9
2.5 2.7 2.9 3.1 3.3 3.5 3.7 3.9
Ag K /
23. Radiochromic film imaging in contact exposure geometry
MD-55-2
0 35
0.45
D
Profile Scan
PeC Microdensitometer
0.15
0.25
0.35
0 2 4 6 8 10
OD
0 2 4 6 8 10
X Position (mm)
24. Air Anisotropy Ratio = A
HPGe
spectrometer
( = 0 )
seed
z
( = 0, )
( = /2, 3/2)
x
y
177 cm Seed rotates in the x-y plane about the z axis
)
2
/
3
(
)
2
/
(
)
(
)
0
(
spec
spec
spec
spec
A
K
K
K
K
E
K
en
i = photon fluence rate
.
i
i
i
i E
K
en
spec Ei = photon energy
(en / )i = mass energy-absorption coefficient
27. Seed with Highly Uniform Emission (A ~ 1)
WAFAC
/ 2 ~ 8o
Well Chamber
~ 4
28. Seed with Highly Directional Emission (A ~ 0)
LOWER
SK
I
WAFAC
/ 2 ~ 8o
Well Chamber
~ 4
more directional less emission “missed” by the WAFAC
that is detected by the well chamber
29. 2 25
103Pd seeds with sphere or pellet design
2.20
2.25
Eav = 20.7 keV
SK
I
2.15
Model
“A”
Model
“B”
Model
“C”
2.05
2.10
/
S
K
(pA
/
U)
2.00
2.05
I
Model
“D”
1.95
D
1.90
30. 2 25
103Pd seeds with sphere or pellet design
2.20
2.25
Eav = 20.7 keV
SK
I
2.15
Model
“A”
Model
“B”
Model
“C”
2.05
2.10
/
S
K
(pA
/
U)
0.33 0.24 0.49
2.00
2.05
I
A Model
“D”
1.95
0.06
D
1.90
31. 125I seed Model “E”
I
4.40
Eav = 27.3 keV
SK
I
4.30
4.35
U)
av
4.20
4.25
I
/
S
K
(pA
/
U
- 2.6 %
Nov03
Sep02
Feb02
4 05
4.10
4.15
Sep02
May04
Feb02
4.05
y
33. Effects of Seed Geometry and Nuclide on A
0.9
1.0
0.7
0.8
0.4
0.5
0.6
A
0 1
0.2
0.3
0.0
0.1
34. Effects of Seed Geometry and Nuclide on A
1 Same Design for 125I and 103Pd Models No End Welds
0.9
1.0
1. Same Design for 125I and 103Pd Models, No End Welds
0.7
0.8
0.4
0.5
0.6
A
125I 103Pd
0 1
0.2
0.3
0.0
0.1
35. Effects of Seed Geometry and Nuclide on A
2 Same Design for 125I and 103Pd Models With End Welds
0.9
1.0
2. Same Design for 125I and 103Pd Models, With End Welds
0.7
0.8
0.4
0.5
0.6
A
0 1
0.2
0.3 125I
103Pd
0.0
0.1
36. Effects of Seed Geometry and Nuclide on A
3 125I Models: Uniform Encapsulation vs End Welds
0.9
1.0
3. 125I Models: Uniform Encapsulation vs. End Welds
(“radiotransparent” substrate)
UE
0.7
0.8
UE
0.4
0.5
0.6
A
0 1
0.2
0.3
EW
0.0
0.1
37. Effects of Seed Geometry and Nuclide on A
4 125I Models w/ Ag: Wire vs Sphere Substrates
0.9
1.0
4. 125I Models w/ Ag: Wire vs. Sphere Substrates
0.7
0.8
0.4
0.5
0.6
A
0 1
0.2
0.3
W
0.0
0.1 W
S
38. Well Chamber Response Coefficients for 22 Seed Models
5.3
5.8
I (pA)
4.8
5.3
131Cs
SK (U)
3.8
4.3
S
K
(pA
/
U)
Ag wires
no Ag
2.8
3.3
I
/
103Pd
125I
Ag spheres
Ag spheres
1.8
2.3
103Pd
39. Photon-emitting sources
g
L 50 k V Hi h
Low-energy < 50 keV < High-energy
LDR HDR LDR HDR
SK SK SK SK, Dw
WAFAC FAC Cavity Cavity, Calorimeter
40. Primary Standard for Low-Energy X Rays (10 kV to 60 kV):
Lamperti Free-Air Chamber
V
HPGe
Miniature x-ray
tube source El t t
tube source
40 kV to 50 kV
Electrometer
41. Primary Standard for Mammography X Rays (≤ 50 kV):
Attix Free-Air Chamber (NIST, University of Wisconsin)
Electrometer
Miniature x-ray
V
Miniature x ray
tube source
40 kV to 50 kV
42. Photon-emitting sources
g
L 50 k V Hi h
Low-energy < 50 keV < High-energy
LDR HDR LDR HDR
SK SK SK SK, Dw
WAFAC FAC Cavity Cavity, Calorimeter
43. Primary Standard for Gamma Rays from 137Cs and 192Ir
1. Graphite-walled Cavity Chambers
)
/
(
)
/
(
1
2
L
d
W
)
(
)
(
)
,
(
)
(
)
/
(
)
/
(
)
/
(
)
/
(
1
1
)
(
2
2
Q
K
Q
K
K
Q
K
M
K
K
L
L
g
V
d
e
W
d
Q
K
S h
wall
stem
det
dr
air
gr
gr
en
air
en
air
air
K
137Cs
1
1 cm3
1 source
192Ir
50 cm3
50 sources
44. Primary Standard for Gamma Rays from 137Cs and 192Ir
2. Spherical Aluminum Cavity and Re-entrant Chambers
2800 cm3
137Cs
US
WS
US I
S
S
2800 cm3
137Cs
Working standard (WS)
or
Unknown source (US)
WS
K
K
I
S
S
U ow sou ce (US)
192Ir
3400 cm3
50
1
i
i
K
US
US
K
I
S
I
S
226Ra source
T1/2 = 1600 y
45. Photon-emitting sources
g
L 50 k V Hi h
Low-energy < 50 keV < High-energy
LDR HDR LDR HDR
SK SK SK SK, Dw
WAFAC FAC Cavity Cavity, Calorimeter
46. Primary Standard for Gamma Rays from HDR 192Ir
Graphite-walled Cavity Chamber, Kair
N i l Ph i l L b (NPL UK)
• National Physical Laboratory (NPL, UK)
• Laboratoire National Henri Becquerel (LNHB, France)
• Physikalisch Technische Bundesanstalt (PTB, Germany)
• Nederlands Meetinstitut (NMi Netherlands)
• Nederlands Meetinstitut (NMi, Netherlands)
• Bhabha Atomic Research Centre (BARC, India)
Water Calorimeter, Dw
• McGill University (Sarfehnia, Stewart, and Seuntjens)
47. Secondary Standard for Gamma Rays from HDR 192Ir
Goetsch “Seven Distance” Technique (1991), AAPM ADCLs
)
(
2
)
(
)
( 250
250
Q
K
N
Q
K
N
Q
K
N Ir
wall
Cs
K
Cs
wall
M
K
M
wall
Ir
K
NIST traceability is achieved through NK
M250 and NK
Cs (cavity chamber)
48. Secondary Standard for Gamma Rays from HDR 192Ir
Goetsch “Seven Distance” Technique (1991), AAPM ADCLs
)
(
2
)
(
)
( 250
250
Q
K
N
Q
K
N
Q
K
N Ir
wall
Cs
K
Cs
wall
M
K
M
wall
Ir
K
NIST traceability is achieved through NK
M250 and NK
Cs (cavity chamber)
“K-weighted Average” Technique, Mainegra-Hing and Rogers (2006)
Ei
K
i
Ir
air
Ei
air
Ir
K N
K
K
N
1
1
2
1
1
1
250
Cs
K
M
K
Ir
K
N
N
N
192Ir ~ M250 + 137Cs
Kair
M250 = Kair
Cs
49. Secondary Standard for Gamma Rays from HDR 192Ir
Accounting for Scattering
1. Goetsch “Seven Distance” Technique
50. Secondary Standard for Gamma Rays from HDR 192Ir
Accounting for Scattering
1. Goetsch “Seven Distance” Technique
51. Secondary Standard for Gamma Rays from HDR 192Ir
Accounting for Scattering
1. Goetsch “Seven Distance” Technique
]
)
,
(
[
)
( scat
det
Ir
K
air M
Q
K
M
N
Q
K
2
)
)(
( c
d
Q
K
S air
K
52. Secondary Standard for Gamma Rays from HDR 192Ir
Accounting for Scattering
1. Goetsch “Seven Distance” Technique
]
)
,
(
[
)
( scat
det
Ir
K
air M
Q
K
M
N
Q
K
2
)
)(
( c
d
Q
K
S air
K
2. Shadow Shield Technique
)
,
( Q
K
Mdet
53. Secondary Standard for Gamma Rays from HDR 192Ir
Accounting for Scattering
1. Goetsch “Seven Distance” Technique
]
)
,
(
[
)
( scat
det
Ir
K
air M
Q
K
M
N
Q
K
2
)
)(
( c
d
Q
K
S air
K
2. Shadow Shield Technique
scat
M
54. What about absorbed dose?
• Dose rate is typically measured using thermoluminescent dosimeters (TLDs)
placed in solid, water-equivalent phantoms at various distances from a seed
• The dose rate at a reference point (1 cm from the seed on the trans erse a is) is
• The dose rate at a reference point (1 cm from the seed on the transverse axis) is
related to the NIST air-kerma strength standard, SK, through a dose-rate constant,
• Uncertainties on a TLD dose rate measurement at 1 cm are typically 4 % (k = 1)
Uncertainties on a TLD dose rate measurement at 1 cm are typically 4 % (k 1),
SK uncertainties are typically 1 % (k = 1), so uncertainty on is about 8 % (k = 2)
Step 1 Step 2
TLD
.
=
D(r0, 0)
1 cm
SK
cGy h-1 U-1
SK measurement D(r0, 0) measurement
. cGy h U
55. TG-43 Formalism
K
S
r
D )
,
( 0
0
)
,
(
)
(
)
,
(
)
,
(
)
,
(
0
0
r
F
r
g
r
G
r
G
S
r
D L
L
L
K
Dose rate in water
)
(r
G 1
2
2
)
4
/
(
)
0
(
L
r
r
G
Geometry Function
D )
(
Dose rate constant (NIST-traceable SK)
sin
)
,
(
Lr
r
GL )
4
/
(
)
0
,
(
L
r
r
GL
Radial Dose Function 2D Anisotropy Function
1
K
S
r
D )
,
( 0
0
)
,
(
)
,
(
)
,
(
)
,
(
)
(
0
0
0
0
0
0
r
G
r
G
r
D
r
D
r
g
X
X
X
Radial Dose Function
)
,
(
)
,
(
)
,
(
)
,
(
)
,
( 0
0
r
G
r
G
r
D
r
D
r
F
L
L
2D Anisotropy Function
r0 = 1 cm
0 = / 2
56. Beta-emitting sources
Dw
.
Extrapolation Chamber
• Ophthalmic applicators
1. Planar (90Sr/Y)
2 Conca e (106R /Rh)
2. Concave (106Ru/Rh)
• IVB seed and line sources (90Sr/Y 32P)
IVB seed and line sources ( Sr/Y, P)
57. Primary Standard for Beta Brachytherapy Sources
Extrapolation Chamber
k
M
k
Q
S
A
e
W
Q
D det
eff
air
0
a
w,
w )
(
'
d
d
)
(
1
)
(
slope of current vs. air gap
max
0
w
col,
w
d
/
)
(
E
E E
S
Q
S
slope of current vs. air gap
max
0
a
col,
w
a
w,
d
/
)
( E
E E
S
Q
S
Bragg-Gray stopping power ratio
63. Extrapolation Chamber Schematic
Electrometer
C ll ti
29.92 pA
Collecting
electrode
Insulating
gap
Water-equivalent plastic
High-voltage
l t d / i d
Air gap=0.15 mm
electrode/window
Source
rent,
pA
Ionization Curr
Air gap, mm
64. Extrapolation Chamber Schematic
Electrometer
19.95 pA
Collecting
electrode
Insulating
gap
Water-equivalent plastic
High-voltage
l t d / i d
Air gap=0.10 mm
electrode/window
Source
rent,
pA
Ionization Curr
Air gap, mm
65. Extrapolation Chamber Schematic
Electrometer
9.99 pA
Collecting
electrode
Insulating
gap
Water-equivalent plastic
High-voltage
l t d / i d
Air gap=0.05 mm
electrode/window
Source
rent,
pA
Ionization Curr
Air gap, mm
66. TG-60, TG-149 Formalism
K
S
r
D )
,
( 0
0
)
(
)
(
)
,
(
)
(
)
(
F
r
G
D
D L
Dose rate in water
)
(r
G 1
2
2
)
4
/
(
)
0
(
L
r
r
G
Geometry Function
)
,
(
)
(
)
,
(
)
,
(
)
,
(
)
,
(
0
0
0
0
r
F
r
g
r
G
r
D
r
D L
L
L
sin
)
,
(
Lr
r
GL )
4
/
(
)
0
,
(
L
r
r
GL
Radial Dose Function 2D Anisotropy Function
NIST-traceable D(r0, 0)
)
,
(
)
,
(
)
,
(
)
,
(
)
(
0
0
0
0
0
0
r
G
r
G
r
D
r
D
r
g
X
X
X
Radial Dose Function
)
,
(
)
,
(
)
,
(
)
,
(
)
,
( 0
0
r
G
r
G
r
D
r
D
r
F
L
L
2D Anisotropy Function
r0 = 2 mm
0 = / 2
67. Measurement Traceability for Brachytherapy Sources
sources
sources
SK , Dw
.
Manufacturer
secondary standard
ADCL
verification for
well-ionization
chambers
sources
verification for
treatment planning
.
Clinic SK
Clinic , Dw
Clinic
68. Clinical Brachytherapy Source Measurements
W ll i i i h b lib d b ADCL
Well-ionization chambers, calibrated by an ADCL
Photon-emitting sources Beta-emitting sources
ADCL
K
Clinic
Clinic
K
I
S
I
S
ADCL
w
Clinic
Clinic
w
I
D
I
D
+ radiochromic film
69. Summary
• Air-kerma-strength standards are currently used for all photon-emitting
brachytherapy sources, realized by free-air and cavity ionization chambers
brachytherapy sources, realized by free air and cavity ionization chambers
• Absorbed-dose-to-water standards are used for all beta-emitting
brachytherapy sources, realized by extrapolation ionization chambers
y py , y p
• Brachytherapy standards are transferred from NIST and the ADCLs
to clinics using well-ionization chambers (radiochromic film for planar
beta-emitting sources)
• Absorbed-dose-to-water measurement methods for high-energy, HDR
h t itti b h th tl d d l t
photon-emitting brachytherapy sources are currently under development
(water calorimetry)
