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

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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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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
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 …
motivation it s got to be fast and it s got to be right
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

90°

manufacturing test fixture mtf
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

carousel plate
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

calibration
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
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

slide8
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
validation
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!

precision of measurements
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

spatial plots for 25 sources
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
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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