NIST Air-Kerma Standard for Electronic Brachytherapy Calibrations - - PowerPoint PPT Presentation

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NIST Air-Kerma Standard for Electronic Brachytherapy Calibrations - - PowerPoint PPT Presentation

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NIST Air-Kerma Standard for Electronic Brachytherapy Calibrations Michael G. Mitch, Ph.D. Leader, Dosimetry Group Radiation Physics Division Physical Measurement Laboratory National Institute of Standards and Technology Disclaimers Certain

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SLIDE 1

NIST Air-Kerma Standard for Electronic Brachytherapy Calibrations

Michael G. Mitch, Ph.D.

Leader, Dosimetry Group Radiation Physics Division Physical Measurement Laboratory National Institute of Standards and Technology

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SLIDE 2

Disclaimers

Certain commercial equipment, manufacturers, instruments, or materials are identified in this presentation in order to specify the experimental procedure adequately. Such identification is for informational purposes only and is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended to imply that the manufacturer, materials, or equipment are necessarily the best available for the purpose. Xoft, Inc. provided funding for the development of the NIST electronic brachytherapy facility and supplied systems for source control.

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SLIDE 3

Develop dosimetric standards for x rays, gamma rays, and electrons based on the SI unit, the gray, 1 Gy ≡ 1 J / kg

kV x rays MV x rays gamma rays electrons x-ray tube linac irradiator linac, Van de Graaff radioactive source (60Co, 137Cs) radioactive source

NIST Dosimetry Group Strategic Element

free-air chamber calorimetry cavity chamber ultrasonic/optical

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SLIDE 4

NIST Standards for Radiation Therapy

  • External beam (60Co, orthovoltage and MV x rays, electrons, protons)
  • Brachytherapy

Low-Energy, Low-Dose-Rate (125I, 103Pd, 131Cs seeds) High-Energy, Low-Dose-Rate (192Ir seeds, 137Cs sources) High-Energy, High-Dose-Rate (192Ir sources) Low-Energy, High-Dose-Rate (miniature x-ray sources)

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SLIDE 5

Safety and efficacy requires accurate treatment planning Dosimetry traceable to primary standards NIST Standards for Radiation Therapy

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SLIDE 6

Dosimetry of X Rays (E < 300 keV)

KERMA = Kinetic Energy Released per unit MAss d Source Material

              

tr tr

d d E m E K

uA f f

incoh incoh pe pe tr

     

photoelectric Compton

transferred to electrons by x rays

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SLIDE 7

http://physics.nist.gov/PhysRefData/Star/Text/ESTAR.html XCOM: Photon Cross Sections Database http://www.nist.gov/pml/data/xcom/index.cfm

Photon and Charged-Particle Data Center

http://www.nist.gov/pml/data/photon_cs/index.cfm

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SLIDE 8

Air kerma can be measured absolutely with a free-air ionization chamber d Source Air Volume

V e W Q K

air air air air

1          

Secondary electrons liberated charge in a given mass of air

Gy kg J kg 1 C J 33.97 C          

Dosimetry of X Rays (E < 300 keV)

KERMA = Kinetic Energy Released per unit MAss

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SLIDE 9

Free-Air Ionization Chamber (E < 300 keV)

        

i i

k V e W Q K

air air air air

1 

Air Kerma

W anode x-ray tube filters

200 400 600 800 1000 20 40

E (keV) Counts

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SLIDE 10

NIST Free-Air Chambers

Lamperti

Ritz

Chamber X-ray tube potential (kV) Plate separation (mm) Plate height (mm) Collector length (mm) Aperture diameter (mm) Air absorption length (mm) Electric field strength (V / cm) Lamperti 10 to 60 40 50 10 5 39 750 Ritz 20 to 100 90 90 70 10 127 55

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SLIDE 11

NIST Electronic Brachytherapy Calibration Facility, v. 1

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SLIDE 12

NIST Electronic Brachytherapy Calibration Facility, v. 1

Control area Maze entry (leaded glass)

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SLIDE 13

NIST Electronic Brachytherapy Calibration Facility, v. 1

leaded glass shield x-ray source Shield, free-air chamber, and spectrometer rotate around source

HPGe spectrometer I

1.5 kV 50 cm

Lamperti free-air chamber

50 cm

  • The Xoft x-ray source can not be continuously rotated (like a brachytherapy seed)
  • Lamperti free-air chamber and HPGe spectrometer rotate around the source
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SLIDE 14

spectrometer free-air chamber x-ray source HV connection source in water cooling catheter leaded glass shield

NIST Electronic Brachytherapy Calibration Facility, v. 1

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SLIDE 15

Ritz Lamperti spectrometer x-ray source

Comparison of Lamperti and Ritz Free-Air Chambers

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SLIDE 16

PROBLEM – Alignment not reproducible

NIST Electronic Brachytherapy Calibration Facility, v. 1

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SLIDE 17

NIST Electronic Brachytherapy Calibration Facility, v. 2

SOLUTION – Optical table for rigid mounting of instruments

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SLIDE 18

NIST Electronic Brachytherapy Calibration Facility, v. 2

SOLUTION – Larger lead-glass surround

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SLIDE 19

NIST Electronic Brachytherapy Calibration Facility, v. 2

Df = 120o

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SLIDE 20

NIST Electronic Brachytherapy Calibration Facility, v. 2

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SLIDE 21

Pulse Height Distribution - Xoft Source at 50 kV

2000 4000 6000 8000 10000 12000 14000 16000 10 20 30 40 50 E (keV) Counts

Fluorescence peaks at 14.9 keV and 16.7 keV are from Y Peaks from 8 keV to 12 keV are from the W anode Y W

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SLIDE 22

 E h E R E S h H d ) , ( ) ( ) (

S(E) is the incident photon spectrum R(E,h), the response function, is the probability per pulse height that a photon incident with energy E will produce a pulse of height h For a photon detector, the measured pulse-height distribution, H(h), is given by

Spectrometry of X-Ray Sources

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SLIDE 23

T(E) is the window-attenuation factor D(E,ε), the energy-deposition spectrum, is the probability per deposited energy that a photon incident with energy E deposits an energy ε in the detector G(ε,h), the intrinsic resolution function, is the probability per pulse height that the deposition of energy ε will give rise to a pulse of height h

    d ) , ( ) , ( ) ( ) , ( h G E D E T h E R

The response function can be written

Spectrometry of X-Ray Sources

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SLIDE 24

The energy-deposition spectrum D(E,ε) depends on the detector dimensions: cylinder of radius r and height z Accurately calculated by Monte Carlo

Spectrometry of X-Ray Sources

For E < 300 keV: D(E,ε) = P0(E,ε) δ(ε-E) + Pxα(E,ε) δ(ε-E+Eα) + Pxβ(E,ε) δ(ε-E+Eβ) + C(E,ε) Photopeak (complete absorption) Ge Kα and Kβ fluorescence x-ray escape Compton continuum

Seltzer, S.M., “Calculated response of intrinsic germanium detectors to narrow x-ray beams with energies up to 300 keV,” Nucl. Instr. Meth. 188, 133-151 (1981).

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SLIDE 25

10 100 1000 10000 10 20 30 40 50

Counts Energy, keV Pulse Height Distribution True Photon Spectrum

Unfolded Spectrum: Xoft source at 50 kV

 E h E R E S h H d ) , ( ) ( ) (

Ge x-ray escape peaks for Y K x rays Y Kβ x rays Y Kα x rays W L x rays

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SLIDE 26

Spectrum of Xoft Source at 50 kV

0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 5 10 15 20 25 30 35 40 45 50 55 60 Normalized Spectrum Photon Energy, keV

Y Ka Y Kb W Lb,g

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SLIDE 27

        

i i

k V e W I K

air air air air

1  

Factor For: Lamperti Ritz 1 kion ion recombination 1.0000 1.0000 2 khumidity humidity of air 0.998 0.998 3 katt attenuation 1.0087 1.0283 4 kel electron loss 1.0008 1.0000 5 ksc photon scatter 0.9987 0.9970 6 kfl fluorescence reabsorption 0.9979 0.9969 7 kbr/(1-g) effects of bremsstrahlung 1.0 1.0 8 kii initial ion 1.0 1.0 9 kdia diaphragm scatter 1.0 1.0 П k1-9 1.0041 1.0200

Free-Air Chamber Correction Factors for Xoft Source at 50 kV

Air-kerma rate at 50 cm

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SLIDE 28

a Determined as the standard deviation of the mean of the measurement. b Typical value for sources measured in 2013/2014

Uncertainty Budget for Xoft Source at 50 kV

Component For: Relative standard uncertainty, % Type A Type B Qnet, Inet net charge or current sQ

a, sI a

0.06 typical value 0.14b W/e mean energy per ion pair

  • 0.15

ρ0 air density 0.01 0.07 Veff effective volume 0.04 0.01 kion ion recombination 0.03 khumidity humidity of air 0.04 katt attenuation

  • 0.11

kel electron loss

  • 0.06

ksc photon scatter

  • 0.03

kfl fluorescence reabsorption

  • 0.05

kbr/(1-g) effects of bremsstrahlung

  • 0.02

kii initial ion

  • 0.04

kdia diaphragm scatter

  • 0.10

kd electric field distortion

  • 0.20

polarity difference 0.02 Combined air kerma 0.15 0.321

U = 0.71 % (k = 2)

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SLIDE 29

Air-Kerma Rate vs. Air-Kerma Strength

2 air air air 2 air

1 ) ( d k V e W I d d K S

i i K

           

 

Air-kerma strength Vacuum d Source Air Volume

Factor Lamperti П k1-9 1.0041 Kvac/Kair conversion to air-kerma strength 1.12

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SLIDE 30

Manufacturer

sources sources well-ionization chambers sources

secondary standard verification for treatment planning

ADCL

Clinic

Measurement Traceability for Brachytherapy Sources

SK

ADCL K Clinic Clinic K

I S I S       

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SLIDE 31

) , ( ) ( ) , ( ) , ( ) , (     r F r g r G r G S r D

L L L K

      

Dose rate in water

K

S r D ) , (

0 

  

Dose Rate Constant (NIST-traceable SK)

 b  sin ) , ( Lr r GL 

1 2 2

) 4 / ( ) , (

  L r r GL

Geometry Function

) , ( ) , ( ) , ( ) , ( ) (     r G r G r D r D r g

X X X

  

Radial Dose Function

) , ( ) , ( ) , ( ) , ( ) , (      r G r G r D r D r F

L L

  

2D Anisotropy Function r0 = 1 cm 0 = p / 2

AAPM TG-43 Formalism for Brachytherapy Dose Calculations

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SLIDE 32

) , ( ) ( ) , ( ) , ( ) , (

50

     r F r g r G r K r D

i i P i cm i

      

Dose rate in water

cm i i

K r D

50

) , (     

Dose Rate Conversion Coefficient (NIST-traceable K50cm)

2

1 ) , ( r r GP  

Geometry Function

) , ( ) , ( ) , ( ) , ( ) (     r G r G r D r D r g

P P i i i

  

Radial Dose Function

) , ( ) , ( ) , (    r D r D r F

i i i

  

2D Anisotropy Function r0 = 1 cm 0 = p / 2

Modified Formalism for Electronic Brachytherapy Sources

Dose-rate conversion coefficient 

DeWerd, Culberson, Micka, and Simiele, Brachytherapy (2015) .

  • TG-43 point-source approximation
  • 2D Anisotropy Function applicable due to polar anisotropy
  • i subscript denotes applicator
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SLIDE 33

AAPM Dosimetric Prerequisites

LDR Brachytherapy

  • Air-kerma strength calibrations traceable to NIST
  • TG-43 parameters published (experimental and Monte Carlo)
  • NIST standard transferred to the ADCLs
  • Annual comparisons between NIST and ADCLs
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SLIDE 34

LDR Brachytherapy

  • Air-kerma strength calibrations traceable to NIST
  • TG-43 parameters published (experimental and Monte Carlo)
  • NIST standard transferred to the ADCLs
  • Annual comparisons between NIST and ADCLs

Electronic Brachytherapy

  • AAPM Task Group proposed

AAPM Dosimetric Prerequisites

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SLIDE 35
  • NIST air-kerma standard for electronic brachytherapy realized
  • Standard transferred to AAPM ADCL using a well chamber
  • Proficiency test with AAPM ADCL completed
  • New calibration service pending: “Well Ionization Chamber

Calibration with Electronic Brachytherapy Sources”

  • Clinical implementation of new standard in progress

Summary

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SLIDE 36

Acknowledgements

  • Xoft, Inc. – funding for the development of the NIST

electronic brachytherapy facility; sources and associated equipment

  • Mel McClelland and Dave Eardley – design, fabrication, and

installation of mechanical, electrical, and electronic systems

  • Ron Tosh – computer control code
  • Jason Walia – spectrometer calibration