MOSFET Dosimeters for 50 kVp Electronic Brachytherapy with the Xoft Axxent Source Steve Axelrod, PhD...
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Transcript of MOSFET Dosimeters for 50 kVp Electronic Brachytherapy with the Xoft Axxent Source Steve Axelrod, PhD...
MOSFET Dosimeters for 50 kVp Electronic Brachytherapy with the Xoft Axxent SourceSteve Axelrod, PhDTom Rusch, PhDXoft, Inc., Fremont, CAwww.xoftinc.com
Copyright © 2005 Xoft, Inc. Confidential - Internal Use Only
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.
Copyright © 2005 Xoft, Inc. Confidential - Internal Use Only
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
Copyright © 2005 Xoft, Inc. Confidential - Internal Use Only
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
Copyright © 2005 Xoft, Inc. Confidential - Internal Use Only
Cumulative Dose and Dose Rate vs. Time
Unfiltered 50 kVp sourceCumulative Dose vs. Time
0.0
1.0
2.0
3.0
4.0
5.0
6.0
14:31 14:38 14:45 14:52 15:00 15:07
Time (hr:min)
Cu
mu
lati
ve D
ose
(G
y)
Ion chbr
MOSFET
Dose Rate vs. Time
0.0000
0.0005
0.0010
0.0015
0.0020
0.0025
14:29 14:36 14:44 14:51 14:58
Time (hr:min)
Do
se R
ate
(Gy/
hal
f se
c)
Ion chbr
MOSFET
Slope is calibration constant, 0.0031 Gy/mV MOSFET noise at 10 seconds ~9%
Copyright © 2005 Xoft, Inc. Confidential - Internal Use Only
Calibration Slope Changes with Spectrum
MOSFET sensitivity is lower when beam is hardened by Al absorber– Line is a simple exponential
MOSFET Calibration Slope vs. Filter Thickness
0.0000
0.0005
0.0010
0.0015
0.0020
0.0025
0.0030
0.0035
0.0040
0.0045
0 0.5 1 1.5 2 2.5 3 3.5 4
Filter Thickness (mm Al)
Cal
ibra
tio
n S
lop
e (G
y/m
V)
Clinical region > 2 mm (~ 20 mm water)
Copyright © 2005 Xoft, Inc. Confidential - Internal Use Only
Calibration Slope Varies Linearly with Absorption Ratio
Absorption ratio is dose ion in chamber with an absorber, divided by dose with no absorber
MOSFET Calibration Slope vs. Attenuation Ratio
y = 0.0017x + 0.0014
0.0000
0.0005
0.0010
0.0015
0.0020
0.0025
0.0030
0.0035
0.0040
0.0045
0.0050
0.00 0.20 0.40 0.60 0.80 1.00 1.20
Attenuation Ratio
Cal
ibra
tio
n S
lop
e (G
y/m
V)
Copyright © 2005 Xoft, Inc. Confidential - Internal Use Only
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