A fixture designed to provide high precision, high throughput routine spatial characterization of the Xoft Axxent™ X-ray source Linda Kelley, Steve Axelrod, Mike Powell Xoft, Inc. Acknowledgements to: Aileen Lum, Long Nguyen, Carol Raleigh, Rick Walkup, Sara Yao, Zirao Zheng

The Xoft Axxent™ X-ray source Miniature x-ray source operating at 50kVp and 300μA 2.2 mm diameter Attached to a flexible high voltage cable and enclosed in a cooling catheter Used in brachytherapy indications Accelerated partial breast irradiation (APBI) Endometrial and rectal Future indications: IORT, superficial … 2.2 x 15 mm bare source 3.5mm diameter cableized 5.3mm cooling sheath outer diameter Cable and controller Applications APBI Endo and rectal IORT Superficial radiation therapy - basal cell carcinoma

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 Polar distribution Azimuthal asymmetry Output level and stability Using a HDR-1000+ well chamber High throughput We have designed, built and validated an apparatus to accomplish these goals 0° 90° To assure an accurate delivery of radiation, must match TG-43 protocol parameters to within an overall error budget Depth dose distribution Polar distribution Azimuthal asymmetry Output level and stability Using a HDR-1000+ well chamber DRC? High throughput to keep up with increased production and more efficiently use operators time

Manufacturing Test Fixture (MTF) Water tank Shielded enclosure with leaded acrylic window Carousel assembly holding all detectors PTW 34013 ion chamber Photodiode devices critical fixtures are Solid WaterTM Shielded bridge and diode amplifiers on top plate Source to be tested is fixed, carousel assembly rotates Source Bridge Assembly Top Plate (raised) Carousel Assembly Acrylic water tank Leaded acrylic and stainless steel enclosure Carousel assembly with all detectors PTW 34013 ion chamber Hamamatsu S8559 Scintillator/photodiodes critical fixtures are Solid WaterTM Source to be tested is fixed, carousel assembly rotates Shielded bridge on top plate with diode amplifiers Water Tank Assembly

Carousel Plate 1cm ion chamber: 1cm depth dose measurement , calibration of all photodiode detectors 2cm C-arm: 6 photodiode detectors: 7cm detector Polar 0°-150° at 30° spacing. 90°: azimuthal and 2cm depth dose 4cm and 7cm photodiodes: depth dose Source holder 1 cm Ion chamber C-arm detectors 4cm detector 1cm: miniature soft x-ray ionization chamber used to measure depth dose can be moved to all other diode distances to calibrate the photodiode detectors 2cm C arm photodiode detectors: 0°,30°,60°,90°,120°,150°. Hamamatsu S8559 x-ray monitor detector designed for x-ray detection below 100keV 5.8mm diameter CsI scintillator custom gain amplifiers for the different distance detectors calibrated in place with the IC measures the polar distribution at 0,30,60,90,120,150 degrees 90 deg – measures the azimuthal deviation 90deg – measures depth dose 4 and 7 cm photodiodes measure depth dose Source holder fixed stop for positioning the source anode Tolerances .002 in enclosures measured at a metrology house or at xoft custom made gauges for in situ measurements actual detector positions entered into software 90°: azimuthal and 2cm depth dose 4cm and 7cm photodiodes: depth dose Position tolerances: .002” - measured and software corrected Alignment assembly Source to test Carousel plate

Calibration Position tolerances: .002” Enclosures measured Custom gauges Calibration of Si photodiodes coupled to CsI scintillator Response dependent on spectra All diodes calibrated in situ Ion chamber is moved to each photodiode distance Distance corrections and calibration constants in software In use position Calibration position Rotated C-arm

Detector paths Detectors read every 20° Source to test 1cm depth dose path Polar 150° path Polar 120° path 2cm depth dose 2cm Azimuthal Polar 90° path Polar 60° path 7cm depth dose path Polar 0° Polar 30° path 4cm depth dose path

Parameters are measured simultaneously with all 9 detectors. Time to acquire data ~ 10 minutes 4 tests per hour Custom LabVIEW software controls all operation Operation of the source Motion of detectors Recording and processing data Printing of test reports >40,000 source lines of code Safety interlocks

Made possible by: Chocolate! Validation Software was validated and is under document control All equipment was extensively validated Installation, operational and performance qualification Validating all recipes Validating an extensive list of normal and abnormal conditions Made possible by: Chocolate!

Sample printed output One of a total of 6 sheets describing the spatial parameters and test conditions Go into the record for each accepted source

Precision of measurements Performance qualification: repeatability, reproducibility, operator to operator variability No operator adjustments to the source or detector position make the measurements highly reproducible. Operator to operator variability was measured to be less than 1% for 3 operators testing 3 sources during performance qualification Performance qualification repeatability – one source tested 5 times without removing it from the fixture reproducability – one source tested 5 times removing it each time from the fixture operator to operator variability – 3 operators testing 3 sources Azimuthal deviation from average Polar distribution averaged over azimuthal angles, normalized at 90deg

Spatial plots for 25 sources Normalized depth dose distributions averaged over azimuthal angle Azimuthal asymmetry as percent deviation from the average. Normalized polar output averaged over azimuthal angle. 0° is along the source axis.

Summary The MTF provides accurate measurement of the polar, azimuthal and depth dose characteristics of Xoft x-ray sources. Measurements are consistent with those made with a previously validated spatial test fixture Measurements have been verified to be highly reproducible Parameters are measured simultaneously resulting in a high throughput of up to 4 sources per hour With adaptation, the apparatus could be used to characterize other small sources