1. MOSFET Dosimeters for 50 kVp Electronic Brachytherapy with the Xoft Axxent Source
Steve Axelrod, PhD
Tom Rusch, PhD
Xoft, Inc., Fremont, CA

2. Background and Motivation
Xoft is introducing a novel miniature x-ray source which is FDA approved for use in HDR breast brachytherapy
Skin dose is increasingly being seen as important in this treatment
MOSFET dosimeters are possibly the most convenient means of measuring skin dose in this situation
The Xoft source operates at 50 kVp with a broad x-ray spectrum
It is inserted into a balloon of ~4cm diameter in the lumpectomy cavity
Response of MOSFETs in this energy range is known to be sensitive to the spectrum, and higher than at mega-voltages.

3. The Measurements
A Xoft source operating at 50 kVp and 300 μA illuminated an Exradin model A600 ion chamber and two Thomson-Nielsen dosimeters, all at 100 mm
MOSFETs were read out with the Thomson-Nielsen supplied program at 10 second intervals
The ion chamber was connected to a UniDos electrometer and read into a custom LabVIEW program on a standard PC, which also controlled the source
A sequence of aluminum absorbers was placed in the beam path to modify the spectrum, as would occur with tissue in clinical use

4. Analysis
Data from both sources was exported to a spreadsheet for comparison
Linearity was established
Calibration constants were assigned to the MOSFET readings to optimize agreement with dose as measured by the ion chamber
Variation in the calibration constants with absorber thickness was studied

5. Cumulative Dose and Dose Rate vs. Time
Unfiltered 50 kVp source
Slope is calibration constant, Gy/mV
MOSFET noise at 10 seconds ~9%

6. Calibration Slope Changes with Spectrum
MOSFET sensitivity is lower when beam is hardened by Al absorber
Line is a simple exponential
Clinical region > 2 mm (~ 20 mm water)

7. Calibration Slope Varies Linearly with Absorption Ratio
Absorption ratio is dose ion in chamber with an absorber, divided by dose with no absorber

8. Conclusion
MOSFET dosimetry can be used to measure skin dose in Xoft Axxent breast brachytherapy
Effects of beam hardening by tissue on the MOSFET calibration should be taken into account
Real time readout in the Thomson-Nielsen system may have future applications