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 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 …


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°


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


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


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


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°


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

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


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


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.


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

Documents

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