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Calibration of Concave Calibration of Concave 106106Ru Applicators at NISTRu Applicators at NIST
Christopher G. SoaresIonizing Radiation Division
National Institute of Standards and Technology
Physics LaboratoryPhysics Laboratory
UNITED STATES DEPARTMENT OF COMMERCE
National Institute of Standards and TechnologyGaithersburg, MD 20899-0001
UNITED STATES DEPARTMENT OF COMMERCE
National Institute of Standards and TechnologyGaithersburg, MD 20899-0001
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HistoryHistory
1937 Fialla builds first extrapolation chamber
1947 Beta-ray applicators in use
1952 Amersham makes first metal foil sources
1953 Loevinger benchmark paper on extrapolation chambers
1976 Establishment of NBS calibration service by Pruitt & Loevinger
1988 Publication of serious discrepancy between NBS and Amersham
1990 Reestablishment of revised NIST calibration service by Soares
1996 Establishment of the UW calibration service by DeWerd
1997 ICRU Report Committee for medical betas formed
1998 First international dosimetry intercomparison for applicators
200? Establishment of national standards at NPL and PTB
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Extrapolation Chamber Measurement GeometriesExtrapolation Chamber Measurement Geometries
30 mm diametercollecting electrode
10 mm diametersource
300 mm longair path
1 mm sourcein TE plastic block
1 mm diameter collecting electrodein contact with block surface
Protection Level
10 mm diametersource
4 mm diameter collecting electrode in contactor at 1 mm in tissue-equivalent plastic
Ocular Therapy
Brachytherapy
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Extrapolation Chamber MeasurementsExtrapolation Chamber Measurementsof Beta-Particle Sourcesof Beta-Particle Sources
Source Geometry Application 1 sigma
near point 10s cm in air protection +2%
planar mm in tissue ocular therapy +6%
seed/line mm in tissue brachytherapy +10%
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Water-equivalent plasticHigh-voltageelectrode/window
Ionization
Collectingelectrode
Insulatinggap
Air gap=0.40 mm
Extrapolation Chamber SchematicExtrapolation Chamber Schematic
Electrometer
79.54 pA
Air gap, mm
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Water-equivalent plasticHigh-voltageelectrode/window
Collectingelectrode
Insulatinggap
Air gap=0.35 mm
Extrapolation Chamber SchematicExtrapolation Chamber Schematic
Electrometer
69.73 pA
Ionization
Air gap, mm
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Water-equivalent plasticHigh-voltageelectrode/window
Collectingelectrode
Insulatinggap
Air gap=0.30 mm
Extrapolation Chamber SchematicExtrapolation Chamber Schematic
Electrometer
59.82 pA
Ionization
Air gap, mm
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Water-equivalent plasticHigh-voltageelectrode/window
Collectingelectrode
Insulatinggap
Air gap=0.25 mm
Extrapolation Chamber SchematicExtrapolation Chamber Schematic
Electrometer
49.85 pA
Ionization
Air gap, mm
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Water-equivalent plasticHigh-voltageelectrode/window
Collectingelectrode
Insulatinggap
Air gap=0.20 mm
Extrapolation Chamber SchematicExtrapolation Chamber Schematic
Electrometer
39.89 pA
Ionization
Air gap, mm
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Water-equivalent plasticHigh-voltageelectrode/window
Collectingelectrode
Insulatinggap
Air gap=0.15 mm
Extrapolation Chamber SchematicExtrapolation Chamber Schematic
Electrometer
29.92 pA
Ionization
Air gap, mm
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Water-equivalent plasticHigh-voltageelectrode/window
Collectingelectrode
Insulatinggap
Air gap=0.10 mm
Extrapolation Chamber SchematicExtrapolation Chamber Schematic
Electrometer
19.95 pA
Ionization
Air gap, mm
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Water-equivalent plasticHigh-voltageelectrode/window
Collectingelectrode
Insulatinggap
Air gap=0.05 mm
Extrapolation Chamber SchematicExtrapolation Chamber Schematic
Electrometer
9.99 pA
Ionization
Air gap, mm
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NIST Medical Extrapolation ChamberNIST Medical Extrapolation ChamberCollecting ElectrodesCollecting Electrodes
4 mm0.6 mm
1.2 mm
Aeff=14.68 mm2 Aeff=0.648 mm2
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[ ]0 D(z0) = . (W/e) Sair
medium
0 A k’
dd
k I()
Extrapolation Chamber Dose EquationExtrapolation Chamber Dose Equation
D(z0) absorbed dose rate in medium at depth z0 (W/e) average energy to create an ion pair (33.97 J/C)
Sair stopping power of medium relative to air (1.12)
0 air density at reference conditions (1.197 kg/m3)
A area of the collecting electrode (14.68 mm2)
I () net current at air gap [ ]0 slope of current vs air gap function at zero air gap
k’ product of corrections which are independent of air gap
k product of corrections which vary with air gap
medium
.
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Determination of Net CurrentDetermination of Net Current
C the external feedback capacitance
U1,2 initial and final voltages on the feedback capacitor
t integration time
I+,- = C(U2-U1)/t
I = (I+ + |I-|)/2
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Corrections to Measured CurrentCorrections to Measured Currentfor Near Geometry Measurementsfor Near Geometry Measurements
Constant during the extrapolation curve measurement:
k’ba difference in electron backscatter between collector and tissue
Varying during the extrapolation curve measurement:
kTp correction to reference conditions of temperature and pressure
kre correction for recombination losses
kdi correction for radiation field divergence
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Near Geometry Divergence EffectNear Geometry Divergence Effect
Less side losses with decreasing air gap
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Calibration Chain at NISTCalibration Chain at NIST
Extrapolation Extrapolation Chamber withChamber with
4 mm Electrode4 mm Electrode
Extrapolation Extrapolation Chamber withChamber with
1 mm Electrode1 mm Electrode
RadiochromicRadiochromicFilmFilm
Source to beSource to beCalibratedCalibrated
Planar Planar 9090Sr/YSr/YReferenceReference
SourceSource
ReferenceReferenceLine SourceLine Source
Well-TypeWell-TypeIonizationIonizationChamberChamber
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ICRU Report Committee:ICRU Report Committee:
Dosimetry of Beta Rays andDosimetry of Beta Rays andLow Energy Photons forLow Energy Photons for
Brachytherapy with Sealed SourcesBrachytherapy with Sealed Sources
Committee Charter:Collection of existing data
New intercomparison of dosimetryof ophthalmic applicators
Commission Sponsors:R. CaswellA. Wambersie
Committee:W.G. CrossH. Järvinen (Chairman)C.G. SoaresS. VynckierK. Weaver
Consultant:D. Flühs
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National Institute of Standards & TechnologyUSA Chris Soares
National Physical LaboratoryUK Tudor Williams
Radiation and Nuclear Safety AuthorityFinland Hannu Järvinen
Catholic University of Louvain, St.-Luc HospitalBelgium Stefaan Vynckier
Algemeen Ziekenhuis MiddelheimBelgium Bob Schaeken
University Hospital of EssenGermany Dirk Flühs
Participants in the Measurement ComparisonParticipants in the Measurement Comparison
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Field ParameterizationField Parameterization
D(z,r) = D(z0,r0) [D(z,r0)/D(z0,r0)] {D(z,r)/D(z,r0)}
D(z0,r0) = reference absorbed dose z0 = 1 mm r0 = 0 mm
[D(z,r0)/D(z0,r0)] = relative central-axis depth dose
{D(z,r)/D(z,r0)} = relative off-axis dose
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Effective Meas. Phys.
Detector Inst. thickness Covering Diam. Diam. ApplicationExtrap. Chamb. NIST 0 0.0016 PET 0.91 500 1,2
4.32Extrap. Chamb. NPL 0 0.0016 PET 4 140 1GAF films NIST 0.007 0 >0.1 >1 1,2,3
0.0017 0GAF film STUK 0.007 0 3 >1 1TLDs UCL 0.3 0 5 5 1,2
1.0 0Alanine AZM 1.2 0 4.9 4.9 1,2Scintillator NIST 0.4 0.2 PE 1 6 1,2,3Scintillator Essen 1 0.02 Al 1 2.55 2,3Silicon diode STUK 0.05 0.375 PMMA 4 7 2Diamond STUK 0.3 0.65 PS 0.5 7.1 2Ion chamb. STUK 0 0.03 PET + 3 61 2 0.2 water . All dimensions in mm Applications: 1= Reference dose rate; 2=Relative central axis depth dose; 3=Relative off axis dose
Detectors Used for the MeasurementsDetectors Used for the Measurements
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Eye PhantomsEye Phantoms
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Planar Film Irradiation GeometryPlanar Film Irradiation Geometry
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Source GeometriesSource Geometries
90Sr planar 106Ru planar 106Ru concave
1 cm
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Tracerlab & ICN/Tracerlab New England Nuclear
Atlantic Research Corp/Atomchem
Manning Research
Technical Operations
3M
Isotope Products Lab/Nucl. Assocs.
Amersham International
Typical Source ProfilesTypical Source Profiles
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Peak Dose RatePeak Dose Rate 230 mGy/s230 mGy/s
Calibrated AverageCalibrated AverageDose Rate 170 mGy/sDose Rate 170 mGy/s
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9090Sr/Y Planar Source Field Profile at 1 mmSr/Y Planar Source Field Profile at 1 mm
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106106Ru/Rh Concave Source Field Profile at 5 mmRu/Rh Concave Source Field Profile at 5 mm
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Results of Reference Dose Rate DeterminationsResults of Reference Dose Rate Determinations
Extrap. Extrap. GAF GAF 0.3mm 0.3mm 0.4mm 1mm 1.2mm Chamb. Chamb. Film Film TLD diamond scin TLD alanineSource (NIST) (NPL) (NIST) (STUK) (UCL) (STUK) (NIST) (UCL) (AZM)
90Sr 0.237 0.260* 0.229 0.258 0.248 --- 0.278 0.308 0.272NEN Gy/s Gy/s Gy/s Gy/s Gy/s Gy/s Gy/s Gy/s0258
106Ru 1.61** 1.82** 1.62 1.63** 1.40 1.99 1.69 1.75 1.49BEBIG mGy/s mGy/s mGy/s mGy/s mGy/s mGy/s mGy/s mGy/s mGy/splanar
106Ru --- --- 2.48 2.10*** 2.55 3.17 --- 2.79 2.89BEBIG mGy/s mGy/s mGy/s mGy/s mGy/s mGy/s mGy/s mGy/s mGy/sconcave
*from a contact measurement with a factor of 0.573 applied to correct to 1 mm** from a contact measurement with a factor of 0.742 applied to correct to 1mm***from a measurement at 1.2 mm with a factor of 1.082 applied to correct to “
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0.00001
0.0001
0.001
0.01
0.1
1
10
0 200 400 600 800 1000 1200
Depth in material /mg cm-2
Re
lati
ve
do
se
ra
te
NIST RCF
0.3 mm TLD
1 mm TLD
Alanine
STUK diode
NIST EC
STUK IC
NIST scin
Essen scin
ACCEPTCode
Depth Dose Measurements of a Depth Dose Measurements of a 9090Sr Ophthalmic ApplicatorSr Ophthalmic Applicator
Gradient at 1 mm:6.5% per 0.1 mm
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0
0.25
0.5
0.75
1
1.25
1.5
1.75
2
0 200 400 600 800 1000 1200
Depth in material /mg cm-2
Rel
ativ
e d
ose
rat
e
NIST RCF
0.3 mm TLD
1 mm TLD
Alanine
STUK diode
NIST EC
STUK IC
NIST scin
Essen scin
ACCEPTCode
Depth Dose Measurements of a Depth Dose Measurements of a 9090Sr Ophthalmic ApplicatorSr Ophthalmic Applicator
Gradient at 1 mm:6.5% per 0.1 mm
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0.0001
0.001
0.01
0.1
1
10
0 400 800 1200 1600
Depth in material / mg cm-2
Re
lati
ve
do
se
ra
te
NIST RCF
1 mm TLD
STUKdiode
NIST scin
Essenscin
STUKdiamond
0.3 mmTLD
Alanine
ACCEPTCode
Depth Dose Measurements of a PlanarDepth Dose Measurements of a Planar106106Ru Ophthalmic ApplicatorRu Ophthalmic Applicator
Gradient at 1 mm:3% per 0.1 mm
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0
0.25
0.5
0.75
1
1.25
1.5
0 400 800 1200 1600
Depth in material / mg cm-2
Re
lati
ve
do
se
ra
te
NIST RCF
1 mm TLD
STUKdiode
NIST scin
Essenscin
STUKdiamond
0.3 mmTLD
Alanine
ACCEPTCode
Depth Dose Measurements of a PlanarDepth Dose Measurements of a Planar106106Ru Ophthalmic ApplicatorRu Ophthalmic Applicator
Gradient at 1 mm:3% per 0.1 mm
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0.0001
0.001
0.01
0.1
1
10
0 400 800 1200 1600
Depth in material / mg cm-2
Re
lati
ve
do
se
ra
te
NIST RCF
0.3 mmTLD
1 mm TLD
Alanine
STUKdiamond
STUKdiode
Essenscin
ACCEPTCode
Depth Dose Measurements of a Curved Depth Dose Measurements of a Curved 106106Ru Ophthalmic ApplicatorRu Ophthalmic Applicator
Gradient at 1 mm:2% per 0.1 mm
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0
0.25
0.5
0.75
1
1.25
0 400 800 1200 1600
Depth in material / mg cm-2
Re
lati
ve
do
se
ra
te
NIST RCF
0.3 mmTLD
1 mm TLD
Alanine
STUKdiamond
STUKdiode
Essenscin
ACCEPTCode
Depth Dose Measurements of a Curved Depth Dose Measurements of a Curved 106106Ru Ophthalmic ApplicatorRu Ophthalmic Applicator
Gradient at 1 mm:2% per 0.1 mm
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Spread in Measurement ResultsSpread in Measurement Results
Source Contact 1 mm 2 mm 3 mm 4 mm 5 mm 7 mm 10 mm 90Sr 6.2% 9.6% 5.3% 3.4% 4.8% 8.8% --- ---NEN (10) (8) (10) (10) (9) (9)0258
106Ru 5.5% 10.5% 4.0% 6.1% 6.4% 6.8% 10% 19%BEBIG (8) (9) (8) (8) (8) (8) (7) (7)planar
106Ru 8.2% 14% 4.8% 6.9% 7.3% 7.0% 9.4% 18% BEBIG (8) (6) (8) (8) (8) (8) (8) (8)concave
All percentages are single standard deviations. Yellow values are for reference absorbed dose rate; white values are for relative central-axis dose.
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Effective Point of MeasurementEffective Point of Measurement
t
z z+t
D(z)
The depth of an infinitely thin detector that gives thesame dose rate as that averaged over a detector of finite thickness, t
Davg(t,z) = D(z) dz
t
z+t
z
If the gradient is constant across the detector, thenthe effective point of measurement is in the center.Otherwise, calculate
and find the value of z which gives the value
Davg(t,z) in the function D(z).
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Measured Off-Axis Dose Function forMeasured Off-Axis Dose Function forthe the 9090Sr/Y Planar sourceSr/Y Planar source
Essen scintillator NIST radiochromic film NIST EGS4 calculation
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Essen scintillator NIST radiochromic film NIST EGS4 calculation
Measured Off-Axis Dose Function forMeasured Off-Axis Dose Function forthe the 106106Ru/Rh Planar sourceRu/Rh Planar source
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Essen scintillator NIST radiochromic film NIST EGS4 calculation
Measured Off-Axis Dose Function for the Measured Off-Axis Dose Function for the 106106Ru/Rh Concave sourceRu/Rh Concave source
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Applicator Types StudiedApplicator Types Studied
CCB
CCX
CIB
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CCB Applicator Dose Profile in Plane 2 mm from CenterCCB Applicator Dose Profile in Plane 2 mm from Center
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CIB Applicator Dose Profile in Plane 4 mm from CenterCIB Applicator Dose Profile in Plane 4 mm from Center
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results
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Distance off axis, mm
Depth, mm 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10 10.5 11
0 1.29
0.5 1.13 1.15 1.17 1.19 1.21 1.23 1.25
1 0.99 0.99 1.00 1.00 1.02 1.06 1.10 1.15 1.19 1.19
1.5 0.87 0.87 0.88 0.90 0.92 0.94 0.97 1.01 1.05 1.12 1.20 1.31
2 0.77 0.78 0.80 0.83 0.86 0.90 0.94 0.98 1.03 1.09 1.14 1.19 1.25 1.31
2.5 0.69 0.70 0.70 0.71 0.71 0.72 0.73 0.75 0.77 0.79 0.82 0.86 0.91 0.98 1.06
3 0.53 0.54 0.56 0.58 0.60 0.62 0.65 0.69 0.73 0.77 0.82 0.88 0.94 1.00 1.06 1.12
4 0.42 0.43 0.43 0.44 0.45 0.47 0.48 0.50 0.53 0.56 0.59 0.63 0.68 0.73 0.79 0.85 0.91 0.97
5 0.30 0.31 0.32 0.33 0.34 0.35 0.37 0.38 0.40 0.43 0.45 0.48 0.51 0.55 0.59 0.64 0.69 0.75 0.82 0.89
7 0.16 0.16 0.16 0.17 0.17 0.18 0.18 0.19 0.19 0.20 0.20 0.20 0.19 0.19 0.18 0.16 0.14 0.12 0.099 0.075 0.055 0.044
10 0.031 0.031 0.032 0.033 0.034 0.035 0.036 0.037 0.038 0.039 0.039 0.038 0.037 0.036 0.034 0.032 0.029 0.025 0.022 0.018 0.014 0.010 0.005
Sample of Off Axis Dosimetry MeasurementsSample of Off Axis Dosimetry Measurementsfor CIB Source Along Non-Cutout Ordinal Radiifor CIB Source Along Non-Cutout Ordinal Radii
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0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
0 1 2 3 4 5 6 7 8
1.25
1.5
1.75
2
2.25
2.5
2.75
3
0 1 2 3 4 5 6 7 8
Along cutout ordinal radius
Along non-cutout ordinal radii
Sample of Off Axis Dosimetry Measurements for CIB SourceSample of Off Axis Dosimetry Measurements for CIB Source
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0
5
10
15
20
25
30
35
40
45
0 2 4 6 8 10 12
Depth in solid water, mm
Sp
eci
fic d
ose
ra
te, m
Gy/
s/M
Bq
/mm
2
CCB 784
CCX 41
CIB 208
ICRU
An Attempt to Make a Depth Dose Curve Independent of Source TypeAn Attempt to Make a Depth Dose Curve Independent of Source Type
plot dose rate per unit activity density
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A Curved-electrode Extrapolation Chamber?A Curved-electrode Extrapolation Chamber?
use the applicator as the high-voltage electrode
Electrometer
9.99 pA
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Source MisalignmentSource Misalignment
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Mechanisms to Check Source UniformityMechanisms to Check Source Uniformity
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