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 …
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

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 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 well chamber
DRC?
High throughput to keep up with increased production and more efficiently use operators time

4. Manufacturing Test Fixture (MTF)
Water tank
Shielded enclosure with leaded acrylic window
Carousel assembly holding all detectors
PTW 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 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

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
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

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 0°
Polar 30° path
4cm depth dose path

8. Parameters are measured simultaneously with all 9 detectors.
Time to acquire data ~ 10 minutes 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. 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!

10. 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

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
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

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