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)

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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