Electronic Brachytherapy Sources Thomas W. Rusch
Educational Objectives Understand key elements of ebx source construction & operation Understand the rationale and methods for air kerma strength calibration Understand methods to characterize and confirm source spatial distribution
Source Construction & Operation
Educational Objectives Understand key elements of ebx source construction & operation Understand the rationale and methods for air kerma strength calibration Understand methods to characterize and confirm source spatial distribution
Axxent X-Ray Source 50 kv operating potential High vacuum x-ray tube technology Output: ~10 Gy/min 1cm into tissue Water cooled Fully disposable device Output described in TG-43 format HV connection Cooling connections X-Ray Tube HV Cable X-Ray Source Tip Detail
Axxent X-Ray Source Components Cable Anode Envelope Filament
Axxent Controller Features Handheld barcode scanner for data entry Display Screen/ Touch Screen Control Retractable Power Cord Wheel brakes for position stability during treatment
Axxent Controller Features High voltage power supply and connector Emergency stop button Well Chamber for source constancy checks Adjustable arm for source pullback (in storage position)
Axxent Applicators Balloon Applicators Vaginal Cylinders Skin-Surface Applicators 3-4 cm Spherical 2.0 cm diameter 1.0 cm diameter 4-5 cm Spherical 2.5 cm diameter 2.0 cm diameter 5-6 cm Spherical 3.0 cm diameter 3.5 cm diameter 5x7 cm Ellipsoidal 3.5 cm diameter 5.0 cm diameter 6x7 cm Ellipsoidal
Educational Objective #1 Understand key elements of ebx source construction & operation Diode x-ray source operates at 50 kv and 0.30 ma Water-cooled in a 5.6 mm diameter catheter Dose distribution is described in the TG-43 format Portable controller operates and positions the source FDA has cleared the source and controller for use anywhere in or on the body where radiation treatment is indicated Applicator choices: balloons for accelerated partial breast irradiation, vaginal cylinders, cones for skin or internal organ surfaces
Air Kerma Strength Characterization
Educational Objectives Understand key elements of ebx source construction & operation Understand the rationale and methods for air kerma strength calibration Understand methods to characterize and confirm source spatial distribution
2-D Brachytherapy Dosimetry Formalism, AAPM TG-43U1 (2004) D(r,θ) = S K Λ g L (r) G L (r,θ) F(r,θ) G L (r 0,θ 0 ) D(r,θ) S K Λ g L (r) G L (r,θ) F(r,θ) dose rate to water at point P(r,θ) air kerma strength dose rate constant radial dose function geometry function (line source approximation) 2-D anisotropy function
Air Kerma Standards National primary standards for air kerma are based on either free-air chambers or cavity ionization chambers Free-air chambers: < 300 kev photons e.g. primary x-ray standards, low-energy brachytherapy sources Cavity ionization chambers: > 300 kev photons e.g. 137 Cs (662 kev) and 60 Co (1250 kev) HDR 192 Ir calibrations are based on an ionization chamber calibrated using 250 kv x-rays and 137 Cs 3/24
Calibration Traceability - Most Low Energy Sources Primary standard (NIST Wide-Angle Free-Air Chamber) Secondary standard (ADCL well ionization chamber) Clinical well ionization chamber
Interim Calibration Traceability UW Attix free-air chamber Primary standard (NIST free-air chamber) Secondary standard (ADCL well ionization chamber) Clinical well ionization chamber
Comparison to 125 I Well chamber was also calibrated at UW using a 6711 125 I seed for interim NIST traceability Conversion coefficients between ebx source and 125 I well chamber calibrations are: Voltage Ratio to 125 I 50 kv 1.780 +/- 3.0% 45 kv 2.172 +/- 2.7% 40 kv 2.894 +/- 4.7% Reference: Application Note 06-01, University of Wisconsin Radiation Calibration Laboratory 21/24
HDR 1000 Plus Well Chamber Special aluminum insert designed by Standard Imaging Source strength is measured with the well chamber prior to treatment
Well Chamber Integrated into the Controller Source in well chamber for calibration Well chamber connected internally to electrometer
Educational Objective #2 Understand the rationale and methods for air kerma strength calibration An interim well chamber calibration is being used while a national standard is developed at NIST The interim method was established at the University of Wisconsin Medical Radiation Research Lab Attix free air chamber and a 6711 125 I seed An integral well chamber is used to check source strength constancy prior to each fraction
Spatial Distribution Characterization
Educational Objectives Understand key elements of ebx source construction & operation Understand the rationale and methods for air kerma strength calibration Understand methods to characterize and confirm source spatial distribution
Axxent Brachytherapy Dosimetry Formalism D(r,θ) = S K Λ g P (r) r 02 r 2 F(r,θ) Simplified geometry function due to L < 1 mm Published g P (r) and F(r,θ) (Rivard et al 2006) All sources are tested for agreement with accepted parameter set Uncertainties are added in quadrature
Motivation - it s got to be fast and it s got to be right! To assure an accurate delivery of radiation, must match TG-43 protocol parameters to within an overall error budget Depth dose distribution within 0.03 of TG-43 reference Polar distribution within 0.10 of TG-43 reference Azimuthal asymmetry 7% Dose rate constant within 7% of nominal Output level and stability Using a HDR-1000+ well chamber 0 90 High throughput An apparatus was designed, built and validated to accomplish these goals
Manufacturing Test Fixture Water tank Shielded enclosure with leaded acrylic window Carousel assembly holding all detectors PTW 34013 ion chamber Photodiode devices Critical fixtures are Solid Water TM Shielded bridge and diode amplifiers on top plate Source to be tested is fixed, carousel assembly rotates Bridge Assembly Top Plate (raised) Carousel Assembly Water Tank Assembly Source
Sample printed output
In the Clinic: Shielded Test Fixture Provides a safe enclosure to evaluate source performance Attaches to controller pullback arm like an applicator Can accept a small pinpoint-style ion chamber Source stability Quantitative pullback comparisons Can accept 12.5 x 12.5 cm sheets of radiochromic film Source spatial distribution in one plane Pullback accuracy
Pretreatment QA: Ion Chamber and Film Completely shielded Ion chamber port and radio-opaque position markers
Educational Objective #3 Understand methods to characterize and confirm source spatial distribution Prior to certification for human use, each source is evaluated in manufacturing test fixtures and must perform within prescribed limits The shielded test fixture allows measurement of relative source spatial characteristics using radiochromic film or an ion chamber
Educational Objectives Understand key elements of ebx source construction & operation Understand the rationale and methods for air kerma strength calibration Understand methods to characterize and confirm source spatial distribution
Acknowledgements Stephen Davis and Prof. Larry DeWerd, University of Wisconsin Medical Radiation Research Lab Linda Kelley and Steve Axelrod, Xoft, Inc. 24/24