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 PowellXoft, Inc.
Acknowledgements to: Aileen Lum, Long Nguyen,
Carol Raleigh, Rick Walkup, Sara Yao, Zirao Zheng
Copyright © 2005 Xoft, Inc. Confidential - Internal Use Only
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 …
Copyright © 2005 Xoft, Inc. Confidential - Internal Use Only
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°
0°
Copyright © 2005 Xoft, Inc. Confidential - Internal Use Only
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
Water Tank Assembly
Carousel Assembly
Top Plate (raised)
Bridge Assembly
Source
Copyright © 2005 Xoft, Inc. Confidential - Internal Use Only
Carousel Plate
1cm ion chamber: 1cm depth dose measurement , calibration of all photodiode detectors
2cm C-arm: 6 photodiode detectors:
Carousel plate
1 cm Ion chamber
7cm detector
C-arm detectors
Source to test
Alignment assembly
4cm detector
Polar 0°-150° at 30° spacing.
– 90°: azimuthal and 2cm depth dose
4cm and 7cm photodiodes: depth dose
Source holder
Copyright © 2005 Xoft, Inc. Confidential - Internal Use Only
Calibration
Rotated C-arm
In use position Calibration position
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
Copyright © 2005 Xoft, Inc. Confidential - Internal Use Only
Detector paths
Polar 150° path
Polar 120° path
Polar 60° path
Polar 30° path
Source to test
1cm depth dose path
2cm depth dose 2cm Azimuthal Polar 90° path
4cm depth dose path
7cm depth dose path
Polar 0°
Detectors read every 20°
Copyright © 2005 Xoft, Inc. Confidential - Internal Use Only
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
Copyright © 2005 Xoft, Inc. Confidential - Internal Use Only
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!
Copyright © 2005 Xoft, Inc. Confidential - Internal Use Only
Sample printed output
Copyright © 2005 Xoft, Inc. Confidential - Internal Use Only
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
Azimuthal deviation from average Polar distribution averaged over azimuthal angles, normalized at 90deg
Copyright © 2005 Xoft, Inc. Confidential - Internal Use Only
Spatial plots for 25 sources
Normalized depth dose distributions averaged over azimuthal angle
Normalized polar output averaged over azimuthal angle. 0° is along the source axis.
Azimuthal asymmetry as percent deviation from the average.
Copyright © 2005 Xoft, Inc. Confidential - Internal Use Only
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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