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

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

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

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%

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

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

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

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

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

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

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

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

NIST Medical Extrapolation ChamberNIST Medical Extrapolation ChamberCollecting ElectrodesCollecting Electrodes

4 mm0.6 mm

1.2 mm

Aeff=14.68 mm2 Aeff=0.648 mm2

[ ]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

.

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

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

Near Geometry Divergence EffectNear Geometry Divergence Effect

Less side losses with decreasing air gap

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

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

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

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

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

Eye PhantomsEye Phantoms

Planar Film Irradiation GeometryPlanar Film Irradiation Geometry

Source GeometriesSource Geometries

90Sr planar 106Ru planar 106Ru concave

1 cm

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

Peak Dose RatePeak Dose Rate 230 mGy/s230 mGy/s

Calibrated AverageCalibrated AverageDose Rate 170 mGy/sDose Rate 170 mGy/s

9090Sr/Y Planar Source Field Profile at 1 mmSr/Y Planar Source Field Profile at 1 mm

106106Ru/Rh Concave Source Field Profile at 5 mmRu/Rh Concave Source Field Profile at 5 mm

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 “

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

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

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

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

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

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

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.

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).

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

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

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

Applicator Types StudiedApplicator Types Studied

CCB

CCX

CIB

CCB Applicator Dose Profile in Plane 2 mm from CenterCCB Applicator Dose Profile in Plane 2 mm from Center

CIB Applicator Dose Profile in Plane 4 mm from CenterCIB Applicator Dose Profile in Plane 4 mm from Center

results

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

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

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

A Curved-electrode Extrapolation Chamber?A Curved-electrode Extrapolation Chamber?

use the applicator as the high-voltage electrode

Electrometer

9.99 pA

Source MisalignmentSource Misalignment

Mechanisms to Check Source UniformityMechanisms to Check Source Uniformity