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Linda Kelley, Steve Axelrod, Mike Powell Xoft, Inc. Acknowledgements to: Aileen Lum, Long Nguyen,

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.

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Linda Kelley, Steve Axelrod, Mike Powell Xoft, Inc. Acknowledgements to: Aileen Lum, Long Nguyen,

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  1. 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

  2. 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 …

  3. 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°

  4. Manufacturing Test Fixture (MTF) Source • 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 Bridge Assembly Top Plate (raised) Carousel Assembly Water Tank Assembly

  5. 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 Alignment assembly Source to test Carousel plate

  6. 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

  7. 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 30° path Polar 0° 4cm depth dose path

  8. 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

  9. 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!

  10. Sample printed output

  11. 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 Polar distribution averaged over azimuthal angles, normalized at 90deg Azimuthal deviation from average

  12. 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.

  13. 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

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