MOSFET Dosimeters for 50 kVp Electronic Brachytherapy with the Xoft Axxent Source

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.


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 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%


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)


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)


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

MOSFET Dosimeters for 50 kVp Electronic Brachytherapy with the Xoft Axxent Source

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