Current Status of Electronic Brachytherapy...
Transcript of Current Status of Electronic Brachytherapy...
October 24, 2014
Current Status of Electronic Brachytherapy Dosimetry
2014 NCCAAPM Fall MeetingLa Crosse, WI
Wes Culberson, PhD, DABRUniversity of Wisconsin – Madison
University of Wisconsin Medical Radiation Research Center (UWMRRC)
October 24, 2014
Current Status of Electronic Brachytherapy Dosimetry
2014 NCCAAPM Fall MeetingLa Crosse, WI
Wes Culberson, PhD, DABRUniversity of Wisconsin – Madison
University of Wisconsin Medical Radiation Research Center (UWMRRC)
Disclosures• UWMRRC receives research support from Xoft, a subsidiary of
iCAD
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Outline1. Electronic Brachytherapy Rationale
2. Overview of Commercial Systems
3. Dosimetry Protocols
4. Establishment of NIST-traceable Standards
5. Current Research in the UWMRRC
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Uses for a Miniature X-Ray Tubes
• Imaging – x-ray radiography
• Handheld x-ray spectrometers
• Vacuum applications
• Electronic brachytherapy (eBt)
The Amp TeK Mini-X
Image from a 60kVp research x-ray tube based on carbon nanotube field emitters in Korea
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Electronic Brachytherapy (EBT) Rationale
• Miniature x-ray sources delivering therapeutic doses of radiation– Brehmsstrahlung x-rays created by targeting electrons onto a high-
Z target (usually gold or tungsten)
• No radionuclides used, thus different regulatory requirements (no radioactive materials license needed)
• Commercial units have energies ranging from 30 – 90kVp
• Adjustable dose rates / tube currents
• Less shielding required due to low energies (compared to 192Ir at least)
• Developed in the late 1980s, ~10 companies have pursued since then
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Two Main Applications• Surface (i.e. skin)
• Interstitial, intracavitary, and intraluminary
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Carl Zeiss INTRABEAM®Oberkochen, Germany
• 30, 40, and 50kVp at 40 A
• FDA cleared for intracranial, IORT, skin, and partial breast applications (using a balloon applicator)
• Gold target
• 1.2 Gy/min at surface of 1.5cm applicator
Images courtesy of www.zeiss.com 8 of 43
Elekta Nucletron Esteya®Stockholm, Sweden
• 70 kVp x-ray source + flattening filter
• Runs at 0.5 – 1.6 mA
• FDA cleared for surface treatments in 2013
• 3.3 Gy/min at skin surface
Images courtesy of www.elekta.com 9 of 43
Xoft Axxent®Freemont, CA
• Disposable 40kVp or 50kVp source• FDA cleared for PBI, skin, and cervical (anywhere “in or on the body where
radiation is indicated”)• 300 A• 1 Gy/min at 3cm• Originally designed as an alternative to HDR 192Ir
Images courtesy of www.xoftinc.com
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XstrahlSurrey, UK
• Not FDA cleared
Photographs of Xstrahl exhibit booth at ASTRO Annual Meeting 2014 11 of 43
Advance X-ray TechnologyBirmingham, MI
• “X-ray Scalpel” – Not FDA cleared
• Microfocus x-ray tube coupled to a capillary optics collimator connected to an insertable tip with a metal target
• Up to 20.2 kVp
• 20Gy/min
Gutman et al., Phys Med Biol 49, 4677-88 (2004)12 of 43
Carbon Nanotube Field Emitters
• Up to 70kVp
• Being developed in Korea
Heo, Kim, Ha, and Cho, “A Vacuum-sealed miniature x-ray tube based on carbon nanotube field emitters”, Nanoscale Research Letters 7: 258, 2012. 13 of 43
Definition of Brachytherapy• Distance?
– Literal Latin translation of brachytherapy is “near” or “short-distance” therapy
– Historically, brachytherapy sources have either been implanted interstitially (inside) or directly on the surface
– eBt units can be implemented interstitially or for surface treatments, but typically are not directly on the surface
– eBt nominal SSDs are ~2.5cm – 6cm
source
Brachytherapy Superficial, SSD 15-25cm
0cm – 6cm SSD
Grenz Ray
Contact Therapy SSD<2cm
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AAPM Protocols• None specifically for eBt
• Concepts based on existing reports– TG-43: Brachytherapy dosimetry formalism (1995, 2004)
– TG-56: Code of practice for brachytherapy (1997)
– TG-59: HDR treatment delivery (1998)
– TG-61: Protocol for calibration of kV beams
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Dosimetry Protocols• AAPM TG43
– For low-energy LDR and HDR interstitial sources
– Source strength is air-kerma strength, SK
– Uses the average of Monte Carlo calculations and measured values to determine the 3-D dose distributions
– Uses lookup tables or functions to apply these results
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Dosimetry Protocols• AAPM TG61 (40-300kVp)
– X-ray tube output standard (measured with a NIST FAC and subsequently transferred by the ADCLs) is air kerma, Kair
– Physicists can use the in-air (<100 kVp) or in-phantom (>100 kVp) measurement method
– Beam quality corrections based on the measured x-ray tube HVL
– Conversion from air-kerma to dose to water achieved by fundamental formulas (mass energy-absorption coefficients, BSFs, etc)
– Uses PDDs to scale the dose
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Dosimetry for Surface Applications
• Compatibility of AAPM TG61
• Measuring air kerma, Kair, is possible
• Distances are very close:– effective point of measurement in the chamber needs to be
considered.
– Stem effect normally close to unity since irradiation conditions are similar to calibration conditions.
• For eBt, Monte Carlo based corrections are necessary
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Dosimetry for Surface Applications• Modified TG61 protocol
• Fulkerson et al. 2014 equations 3 and 4
R.K.Fulkerson, J.A. Micka, L.A. DeWerd, “Dosimetric characterization and output for conical brachytherapy surface applicators. Part 1. Electronic brachytherapy source”, Medical physics, 2014, Vol.41(2), pp.022103
• Special holders designed
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Dosimetry for Interstitial, Intraluminary, and Intracavitary
• Compatibility with TG43
• Air-kerma rate vs air-kerma strength
– SK defined in vacuo• Must correct for attenuation in air
• Calculated Xoft Axxent® spectrum by Dr. Steve Davis 2009
• Difficult to rotate the source
• Energies too high for NIST WAFAC
PMMA holder
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Dosimetry for Interstitial, Intraluminary, and Intracavitary
• Dr. Steve Davis measured and calculated the air-kerma strength of an electronic brachytherapy source for his PhD dissertation and determined k=2 uncertainties of 14%– Correcting measurement for filtration in air to account for the entire
spectrum
– Large uncertainties were due to source-to-source variations and uncertainties in the Monte Carlo simulations
• Low energies aren’t clinically relevant, much as in the 4.5 keV Tix-rays around common LDR I-125 and Pd-103 sources
• Filter - air
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Establishing NIST-Traceable Standards
• Xoft, Inc. source had a contract with the UWMRRC and subsequently NIST to establish standards for the S700 source
• Initially, went with the AAPM TG43 approach– Sk difficult to measure– Used a hybrid TG43 formalism with AKR @ 1m as the source strength
metric– Conversion to dose is achieved with a conversion coefficient
• Source strength based on air-kerma rate at 1m in air (not in vacuo) measured with the UW Attix FAC
• Conversion to absorbed dose to water based on measurements and Monte Carlo calculations at UWMRRC
• In 2014, NIST introduced a new source strength metric of air-kerma rate in air at 50cm
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The NIST Standard for the XOFT Axxent® S700 Source
• Introduced in 2013
• Lamperti Free-Air Chamber (FAC)
• Originally intended to swing the FAC around the source
• Now fixed FAC position
X-ray source
FAC
HPGe spectrometer
Image from NIST.gov
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UW Attix FAC• UWMRRC also measured the output with the Attix FAC and
compared with NIST
• Collimation slightly different than Lamperti FAC
• Measured at four cardinal angles
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NIST and UW FAC AgreementXoft Axxent® S700 SourcesRound s/n UW FAC
(Gy/s)NIST FAC
(Gy/s) % Diff
1914160 1.890E-04 1.960E-04 -3.6%914219 1.896E-04 1.610E-04 16.3%914230 1.926E-04 1.790E-04 7.3%
2
914552 1.837E-04 1.920E-04 -4.4%924107 1.788E-04 1.860E-04 -3.9%924137 1.824E-04 1.710E-04 6.4%924275 1.923E-04 1.990E-04 -3.4%
3914533 1.712E-04 1.670E-04 2.5%914568 2.017E-04 2.110E-04 -4.5%924201 2.021E-04 1.950E-04 3.6%
• Measurement angles and air attenuation corrections are likely main sources of discrepancy in the first two rounds
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Azimuthal Angular Dependence
• FAC results as a function of angle
• Source s/n 914219
Azimuthal Angle Attix FAC Response Relative to Zero Degrees
0 1.00090 1.097
180 1.110270 1.067
Attix FAC
eBt source(top view)
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Calibration of Well Chambers• Since clinical users don’t have a FAC, must use well chamber
• Should provide a consistent transfer of AKR to charge readings in the well chamber
– Should give clinically relevant measure of source strength• 4π geometry
• Filter out the low energies (like air and tissue)
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The Well Chamber• Standard Imaging HDR 1000 Plus with a specific insert with
~3mm thick Al holder – filters out the lower energies
• Uses a plastic standoff to position the source in the sweet spot (point of maximum response)
• SNR– Great!
– ~100nA ionization current for Xoft Axxent® S700 source (with special insert)
– For reference, well chamber current an LDR seed in its normal insert is ~9 pA and the UW VAFAC is ~50fA
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The Well Chamber
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Setting up an ADCL Service• Typical well chamber calibration coefficients
– Primary air-kerma measurement performed by NIST on FAC for multiple sources
– Sources sent to a ADCL and measured in a well chamber.
– Ratios of air-kerma to well chamber current used as calibration coefficient
– Hoping for tight range of coefficients
– Coefficients will vary slightly for LDR sources (+/- 2%) due to internal construction of sources
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Source-to-Source Variations
• Not all tungsten targets made the same, especially for these sizes
• Effects of spectral differences– Will affect conversion from AKR to dose to water
– Low energy component of spectrum will be the main issue
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Initial UWADCL Well Chamber ResultsXoft Axxent® Sources
Average of Round 3
values used as the final cal
coefficient
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Implementation of a New Standard
• UWADCL calibrations for the XOFT Axxent® Model S700 source approved by AAPM CLA in summer, 2014
• Slight modification of TG43 is needed to accommodate the new source strength metric of AKR at 50cm in air
• Manuscript submitted by DeWerd et al. to Medical Physics Journal– Proposes a formalism to use the new NIST standard
– Proposes the Dose Conversion Coefficient, χ
– Proposes applicator specific values (not in TG43)
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Current Research in the UWMRRC
• Measurement of dose surrounding eBt sources
• Applicators and their effect on eBt dosimetry
• Relative biological effectiveness (RBE)
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Measuring Dose Around an eBt Source
35Photograph courtesy of Sam Simiele 35 of 43
Applicators• Common for brachytherapy intracavitary treatments
• For 192Ir, metal applicators disrupt the dose distribution minimally
• Electronic brachytherapy will attenuate by a factor of 8!
• All lookup values should be applicator specific
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Applicators
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Measured vs Calculated Results
• Active area of research at UWMRRC by Sam Simiele
• Both bare and in applicator results show substantial disagreement between measured and predicted dose distributions– Difficult to identify the source of the discrepancy
– Monte Carlo?
– TLD energy dependence?
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RBE• RBE of eBt is under scrutiny
• Lower energy, longer treatment times (~10-15 min for IORT)
192Ir Xoft Axxent®
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RBE• Recall, RBE depends on LET
• LET of 10keV x-rays is 10x higher than 1MeV x-rays
• RBE decreases with depth due to beam hardening– Estimated to vary by a factor of 1.5 by Brenner et al. 1999
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Conclusions• Recent years have seen a surge of new eBt manufacturers
• Current AAPM dosimetry protocols need revisions before being implemented for eBt
• NIST-traceable air-kerma standards have been established for the Xoft Axxent® Model S700 source
• Research is underway in the UWMRRC to solve some of the remaining challenges
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Acknowledgements• UWMRRC students and staff
• UWADCL customers for their continued support
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Questions
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