Calibration of Concave 106 Ru Applicators at NIST Christopher G. Soares Ionizing Radiation Division National Institute of Standards and Technology Physics Laboratory UNITED STATES DEPARTMENT OF COMMERCE National Institute of Standards and Technology Gaithersburg, MD UNITED STATES DEPARTMENT OF COMMERCE National Institute of Standards and Technology Gaithersburg, MD

History 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 Geometries 30 mm diameter collecting electrode 10 mm diameter source 300 mm long air path 1 mm source in TE plastic block 1 mm diameter collecting electrode in contact with block surface Protection Level 10 mm diameter source 4 mm diameter collecting electrode in contact or at 1 mm in tissue-equivalent plastic Ocular Therapy Brachytherapy

Extrapolation Chamber Measurements of Beta-Particle Sources Source Geometry Application 1 sigma near point10s cm in airprotection +2% planarmm in tissueocular therapy +6% seed/linemm in tissuebrachytherapy +10%

Water-equivalent plastic High-voltage electrode/window Ionization Collecting electrode Insulating gap Air gap=0.40 mm Extrapolation Chamber Schematic Electrometer pA Air gap, mm

Water-equivalent plastic High-voltage electrode/window Collecting electrode Insulating gap Air gap=0.35 mm Extrapolation Chamber Schematic Electrometer pA Ionization Air gap, mm

Water-equivalent plastic High-voltage electrode/window Collecting electrode Insulating gap Air gap=0.30 mm Extrapolation Chamber Schematic Electrometer pA Ionization Air gap, mm

Water-equivalent plastic High-voltage electrode/window Collecting electrode Insulating gap Air gap=0.25 mm Extrapolation Chamber Schematic Electrometer pA Ionization Air gap, mm

Water-equivalent plastic High-voltage electrode/window Collecting electrode Insulating gap Air gap=0.20 mm Extrapolation Chamber Schematic Electrometer pA Ionization Air gap, mm

Water-equivalent plastic High-voltage electrode/window Collecting electrode Insulating gap Air gap=0.15 mm Extrapolation Chamber Schematic Electrometer pA Ionization Air gap, mm

Water-equivalent plastic High-voltage electrode/window Collecting electrode Insulating gap Air gap=0.10 mm Extrapolation Chamber Schematic Electrometer pA Ionization Air gap, mm

Water-equivalent plastic High-voltage electrode/window Collecting electrode Insulating gap Air gap=0.05 mm Extrapolation Chamber Schematic Electrometer 9.99 pA Ionization Air gap, mm

NIST Medical Extrapolation Chamber Collecting Electrodes 4 mm 0.6 mm 1.2 mm A eff =14.68 mm 2 A eff =0.648 mm 2

[ ]  0 D(z 0 ) =. (W/e) S air medium  0 A k’ d d k I( ) Extrapolation Chamber Dose Equation D(z 0 ) absorbed dose rate in medium at depth z 0 (W/e) average energy to create an ion pair (33.97 J/C) S air stopping power of medium relative to air (1.12)  0 air density at reference conditions (1.197 kg/m 3 ) A area of the collecting electrode (14.68 mm 2 ) 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 Current Cthe external feedback capacitance U 1,2 initial and final voltages on the feedback capacitor tintegration time I +,- = C(U 2 -U 1 )/t I = (I + + |I - |)/2

Corrections to Measured Current for 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: k Tp correction to reference conditions of temperature and pressure k re correction for recombination losses k di correction for radiation field divergence

Near Geometry Divergence Effect Less side losses with decreasing air gap

Calibration Chain at NIST Extrapolation Chamber with 4 mm Electrode 4 mm Electrode Extrapolation Chamber with 1 mm Electrode 1 mm Electrode RadiochromicFilm Source to be Calibrated Planar 90 Sr/Y ReferenceSource Reference Line Source Well-TypeIonizationChamber

ICRU Report Committee: Dosimetry of Beta Rays and Low Energy Photons for Brachytherapy with Sealed Sources Committee Charter: Collection of existing data New intercomparison of dosimetry of ophthalmic applicators Commission Sponsors: R. Caswell A. Wambersie Committee: W.G. Cross H. Järvinen (Chairman) C.G. Soares S. Vynckier K. Weaver Consultant: D. Flühs

National Institute of Standards & Technology USAChris Soares National Physical Laboratory UKTudor Williams Radiation and Nuclear Safety Authority FinlandHannu Järvinen Catholic University of Louvain, St.-Luc Hospital BelgiumStefaan Vynckier Algemeen Ziekenhuis Middelheim BelgiumBob Schaeken University Hospital of Essen GermanyDirk Flühs Participants in the Measurement Comparison

Field Parameterization D(z,r) = D(z 0,r 0 ) [D(z,r 0 )/D(z 0,r 0 )] {D(z,r)/D(z,r 0 )} D(z 0,r 0 ) = reference absorbed dose z 0 = 1 mm r 0 = 0 mm [D(z,r 0 )/D(z 0,r 0 )] = relative central-axis depth dose {D(z,r)/D(z,r 0 )} = relative off-axis dose

Effective Meas. Phys. Detector Inst. thickness Covering Diam. Diam. Application Extrap. Chamb.NIST PET , Extrap. Chamb.NPL PET41401 GAF filmsNIST0.0070>0.1>11,2, GAF filmSTUK >11 TLDsUCL , Alanine AZM ,2 ScintillatorNIST PE161,2,3 ScintillatorEssen10.02 Al12.552,3 Silicon diodeSTUK PMMA472 Diamond STUK PS Ion chamb.STUK00.03 PET 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 Measurements

Eye Phantoms

Planar Film Irradiation Geometry

Source Geometries 90 Sr planar 106 Ru planar 106 Ru 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 Profiles

Peak Dose Rate 230 mGy/s 230 mGy/s Calibrated Average Dose Rate 170 mGy/s

90 Sr/Y Planar Source Field Profile at 1 mm

106 Ru/Rh Concave Source Field Profile at 5 mm

Results 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 alanine Source (NIST) (NPL) (NIST) (STUK) (UCL) (STUK) (NIST) (UCL) (AZM) 90 Sr * NEN Gy/s Gy/s Gy/s Gy/s Gy/s Gy/s Gy/s Gy/s Ru 1.61** 1.82** ** BEBIG mGy/s mGy/s mGy/s mGy/s mGy/s mGy/s mGy/s mGy/s mGy/s planar 106 Ru *** BEBIG mGy/s mGy/s mGy/s mGy/s mGy/s mGy/s mGy/s mGy/s mGy/s concave *from a contact measurement with a factor of applied to correct to 1 mm ** from a contact measurement with a factor of applied to correct to 1mm ***from a measurement at 1.2 mm with a factor of applied to correct to “

Depth Dose Measurements of a 90 Sr Ophthalmic Applicator Gradient at 1 mm: 6.5% per 0.1 mm

Depth Dose Measurements of a 90 Sr Ophthalmic Applicator Gradient at 1 mm: 6.5% per 0.1 mm

Depth Dose Measurements of a Planar 106 Ru Ophthalmic Applicator Gradient at 1 mm: 3% per 0.1 mm

Depth Dose Measurements of a Planar 106 Ru Ophthalmic Applicator Gradient at 1 mm: 3% per 0.1 mm

Depth Dose Measurements of a Curved 106 Ru Ophthalmic Applicator Gradient at 1 mm: 2% per 0.1 mm

Depth Dose Measurements of a Curved 106 Ru Ophthalmic Applicator Gradient at 1 mm: 2% per 0.1 mm

Spread in Measurement Results Source Contact 1 mm 2 mm 3 mm 4 mm 5 mm 7 mm 10 mm 90 Sr 6.2% 9.6% 5.3% 3.4% 4.8% 8.8% NEN (10) (8) (10) (10) (9) (9) Ru 5.5% 10.5% 4.0% 6.1% 6.4% 6.8% 10% 19% BEBIG (8) (9) (8) (8) (8) (8) (7) (7) planar 106 Ru 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 Measurement t zz+t D(z) The depth of an infinitely thin detector that gives the same dose rate as that averaged over a detector of finite thickness, t D avg (t,z) =   D(z) dz t z+t z If the gradient is constant across the detector, then the effective point of measurement is in the center. Otherwise, calculate and find the value of z which gives the value D avg (t,z) in the function D(z).

Measured Off-Axis Dose Function for the 90 Sr/Y Planar source Essen scintillator NIST radiochromic film NIST EGS4 calculation

Measured Off-Axis Dose Function for the 106 Ru/Rh Planar source

Essen scintillator NIST radiochromic film NIST EGS4 calculation Measured Off-Axis Dose Function for the 106 Ru/Rh Concave source

Applicator Types Studied CCB CCX CIB

CCB Applicator Dose Profile in Plane 2 mm from Center

CIB Applicator Dose Profile in Plane 4 mm from Center

results

Sample of Off Axis Dosimetry Measurements for CIB Source Along Non-Cutout Ordinal Radii

Along cutout ordinal radius Along non-cutout ordinal radii Sample of Off Axis Dosimetry Measurements for CIB Source

An Attempt to Make a Depth Dose Curve Independent of Source Type plot dose rate per unit activity density

A Curved-electrode Extrapolation Chamber? use the applicator as the high-voltage electrode Electrometer 9.99 pA

Source Misalignment

Mechanisms to Check Source Uniformity