Implantable MOSFET dosimeter response to 192 Ir HDR radiation

Transcription

1 Implantable MOSFET dosimeter response to 192 Ir HDR radiation Jessica Fagerstrom University of Wisconsin Madison, Department of Medical Physics Medical Radiation Research Center North Central Chapter AAPM meeting Mayo Clinic, Rochester, MN April 25, 2008

2 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) (SiO 2 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.

3 Outline The goal of this study was to characterize the DVS response to 192 Ir HDR sources Dose rate dependence Energy dependence Accumulated dose effect Rotational angular dependence Longitudinal angular dependence

4 Dose Verification System 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, Photograph courtesy of Sicel Technologies, Inc.

5 Virtual Water phantoms: Phantom 1 30 cm 30 cm 15 cm

6 Virtual Water phantoms: Phantom 2 30 cm 30 cm 11 cm

7 Virtual Water phantoms: Phantom 2 Longitudinal testing Rotational/accumulated dose testing

8 Dose rate dependence Dosimeters irradiated in Phantom 1 with 60 Co 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 192 Ir in subsequent tests Delivered dose determined using a NISTcalibrated Exradin A12 ion chamber inserted into the phantom 200 cgy/fraction for nine fractions, 1 fraction/day

9 Dose rate dependence

10 Energy dependence Dosimeters irradiated in Phantom 1 with 60 Co and 192 Ir Fractions alternated between 60 Co and 192 Ir 60 Co delivered dose determined using a NISTcalibrated Exradin A12 ion chamber inserted into the phantom 192 Ir delivered dose was determined using measured S K and TG-43U1 1 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, (2004).

11 Energy dependence

12 Accumulated dose effect Dosimeters irradiated in Phantom 2 with 192 Ir 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

13 Accumulated dose effect ΔV th /D delivered = a + b D acc + c D acc2 + d D acc 3 R 2 >0.997

14 Rotational angular dependence Dosimeters irradiated in Phantom 2 with 192 Ir 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

15 Rotational angular dependence

16 Longitudinal angular dependence Dosimeters irradiated in Phantom 2 with 192 Ir 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

17 Longitudinal angular dependence

18 Uncertainty summary Test Isotope Sensitivity uncertainty Dose rate dependence 60 Co ±4.6% to ±6.3% Energy dependence 60 Co ±2.8% to ±5.1% 192 Ir ±4.1% to ±6.1% Accumulated dose effect 3 cm 192 Ir ±3.2% to ±3.5% 4 cm 192 Ir ±4.0% to ±7.2% 5 cm 192 Ir ±3.3% to ±6.6% Rotational angular dependence 192 Ir ±4.3% to ±7.7% Longitudinal angular dependence 192 Ir ±4.2% to ±5.6%

19 Conclusions Dose rate changes at 60 Co did not have a statistically significant effect on detectors DVS was more sensitive to 192 Ir than 60 Co Higher interaction cross section in SiO 2 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

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