Implantable MOSFET dosimeter response to 192Ir HDR radiation

Jessica Fagerstrom

University of Wisconsin – Madison, Department of Medical PhysicsMedical Radiation Research Center

North Central Chapter AAPM meetingMayo Clinic, Rochester, MN

April 25, 2008

Dose Verification System®

• Implantable dosimeters can verify the sum contribution of many factors.

• Sicel Technologies, Inc. has developed a compact, implantable MOSFET Dose Verification System® (DVS) (SiO2 active area).

• DVS detectors are surgically implanted in or near irradiated areas of patient.

• Dose is read telemetrically by a handheld antenna, which inductively powers the dosimeter.

• DVS received FDA 510(k) clearance in 2006 for external beam breast and prostate treatments.

• DVS is currently characterized for 6 and 18 MV linear accelerators.

Outline• The goal of this study was to characterize the

DVS response to 192Ir HDR sources– Dose rate dependence– Energy dependence

– Accumulated dose effect

– Rotational angular dependence

– Longitudinal angular dependence

Dose Verification System

Photograph courtesy of Sicel Technologies, Inc.

G. P. Beyer et al. , “An implantable MOSFET dosimeter for the measurement of radiation dose in tissue during cancer therapy,” IEEE Sensors J., Vol. 8, No. 1, 2008.

Virtual Water phantoms: Phantom 1

30 cm

30 cm

15 cm

Virtual Water phantoms: Phantom 2

30 cm

30 cm11 cm

Virtual Water phantoms: Phantom 2

Longitudinal testing Rotational/accumulated dose testing

Dose rate dependence• Dosimeters irradiated in Phantom 1 with 60Co

source• SDD varied to achieve dose rates varying

between 22 cGy/min to 84 cGy/min• Dose rates chosen to cover range of those

delivered by 192Ir in subsequent tests• Delivered dose determined using a NIST-

calibrated Exradin A12 ion chamber inserted into the phantom

• 200 cGy/fraction for nine fractions, 1 fraction/day

Dose rate dependence

Energy dependence• Dosimeters irradiated in Phantom 1 with 60Co

and 192Ir• Fractions alternated between 60Co and 192Ir• 60Co delivered dose determined using a NIST-

calibrated Exradin A12 ion chamber inserted into the phantom

• 192Ir delivered dose was determined using measured SK and TG-43U11 methodology

• 600 cGy/fraction for a total of 10 fractions[1] M.J. Rivard, B.M. Coursey, L.A. DeWerd, W.F. Hanson, M.S. Huq, G.S. Ibbott, M.G. Mitch, R. Nath, and J.F. Williamson, “Update of AAPM Task Group No. 43 Report: A revised AAPM protocol for brachytherapy dose calculations,” Med. Phys. 31, 633-674 (2004).

Energy dependence

Accumulated dose effect

• Dosimeters irradiated in Phantom 2 with 192Ir• All inserts oriented at 0°• Dosimeters placed at 3 distances

from source:– 3 cm (1150 cGy/fraction)– 4 cm (650 cGy/fraction)– 5 cm (415 cGy/fraction)

• 10 fractions delivered to all detectors, 1 fraction/day for 2 weeks

Accumulated dose effect

ΔVth/Ddelivered = a + b·Dacc + c·Dacc2 + d·Dacc

3

R2>0.997

Rotational angular dependence• Dosimeters irradiated in Phantom 2 with 192Ir• Varied rotational radiation incidence (about the

long axis of DVS)• Dosimeters placed 4 cm from source• 215 cGy/fraction for 10 fractions,

1 fraction/day for 2 weeks• Irradiations performed at 0°, 45°,

90°, 225°, and 270°• Fractions alternated between 0°

and other angles

Rotational angular dependence

Longitudinal angular dependence

• Dosimeters irradiated in Phantom 2 with 192Ir• Varied longitudinal radiation incidence (about

the short axis of DVS)• Dosimeters placed 10 cm from source• 190 cGy/fraction for 10 fractions,

1 fraction/day for 2 weeks• Irradiations performed at 0°, 30°,

60°, 90°, and 270°• Fractions alternated between 0°

and other angles

Longitudinal angular dependence

Uncertainty summaryTest Isotope Sensitivity uncertainty

Dose rate dependence 60Co ±4.6% to ±6.3%60Co ±2.8% to ±5.1%192Ir ±4.1% to ±6.1%

3 cm 192Ir ±3.2% to ±3.5%

4 cm 192Ir ±4.0% to ±7.2%

5 cm 192Ir ±3.3% to ±6.6%

Rotational angular dependence 192Ir ±4.3% to ±7.7%

Longitudinal angular dependence 192Ir ±4.2% to ±5.6%

Accumulated dose effect

Energy dependence

Conclusions• Dose rate changes at 60Co did not have a statistically

significant effect on detectors• DVS was more sensitive to 192Ir than 60Co

– Higher interaction cross section in SiO2 at an average energy of 397 keV vs 1.25 MeV

• Accumulated dose effect can be characterized with third order polynomials

• Little angular dependence except where detector is irradiated with coil/electronics assembly between MOSFETs and incident radiation– Problem minimized if implanted with coil facing away from

nearest catheter

Acknowledgements

• John Micka• Larry DeWerd• Sicel Technologies• UW MRRC students and staff• UW ADCL customers