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

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.

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

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

Cumulative Dose and Dose Rate vs. Time Unfiltered 50 kVp source Slope is calibration constant, 0.0031 Gy/mV MOSFET noise at 10 seconds ~9%

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)

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

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