small-field studies in proton therapy beams with a gem-based dose imaging detector
DESCRIPTION
A.V. Klyachko 1 , D.F. Nichiporov 1 , L. Coutinho 2 , C.-W. Cheng 2, 3 , M. Luxnat 1 , I. J. Das 2, 3 1 Indiana University Cyclotron Operations, Indiana University Integrated Science and Accelerator Technology Hall, Bloomington, Indiana, USA. - PowerPoint PPT PresentationTRANSCRIPT
Alexander Klyachko, IU Cyclotron, PTCOG51, Seoul, South Korea, May 17-19, 2012
Small-Field Studies in Proton Therapy Beams with a
GEM-based Dose Imaging Detector
A.V. Klyachko1, D.F. Nichiporov1, L. Coutinho2, C.-W. Cheng2, 3, M. Luxnat1, I. J. Das2, 3
1 Indiana University Cyclotron Operations, Indiana University Integrated Science and
Accelerator Technology Hall, Bloomington, Indiana, USA.
2 Indiana University Health Proton Therapy Center, Bloomington, Indiana, USA
3 Indiana University School of Medicine, Indianapolis, Indiana, USA
Alexander Klyachko, IU Cyclotron, PTCOG51, Seoul, South Korea, May 17-19, 2012
Outline: Why small fields?
Why GEM detector?
GEMs in dose imaging – basic principles, optical readout, detector design
Test results
Summary
Alexander Klyachko, IU Cyclotron, PTCOG51, Seoul, South Korea, May 17-19, 2012
Why Small Field Dosimetry:
Small fields (diameter <3 cm) are already in use:- intracranial lesions, base of skull tumors - ophthalmic- patch fields
Accuracy of treatment planning is not well established Dosimetry of small fields is challenging, uncertainties in dosimetry of 10-15 % and up are possible, especially in lateral distributions Lack of adequate detectors for small field measurements does not alleviate the problem.
Alexander Klyachko, IU Cyclotron, PTCOG51, Seoul, South Korea, May 17-19, 2012
WET=0
Beam range 16 cm in water
8 cm
15 cm
15.8 cm
Alexander Klyachko, IU Cyclotron, PTCOG51, Seoul, South Korea, May 17-19, 2012
Gas Electron Multipliers = GEMs (Sauli 1997) show promise to be free of those drawbacks
Why GEMs:
Nonlinear dose and energy response Long measuring time for obtaining complete 2D dose distributions Insufficient spatial resolution Tissue non-equivalence Or a combination thereof
Existing detectors used in clinical practice all have notable shortcomings when applied to small field dosimetry:
fast performance robustness and design flexibility excellent spatial resolution cascade option to improve signal-to-noise ratio electronic and optical readout schemes
Alexander Klyachko, IU Cyclotron, PTCOG51, Seoul, South Korea, May 17-19, 2012
Optical Readout of GEMs
J.H. Timmer et al, A scintillating GEM for 2D-dosimetry in radiation therapy. NIM A478 (2002) 98
F.A.F. Fraga et al, Luminescence and imaging with gas electron multipliers. NIM A513 (2003) 379
S. Fetal et al, Dose imaging in radiotherapy with an Ar-CF4 filled scintillating GEM. NIM A513 (2003) 42
E. Seravalli et al, 2D dosimetry in a proton beam with a scintillating GEM detector . Phys. Med. Biol. 54 (2009) 3755
A.V. Klyachko et al, Dose imaging detectors for radiotherapy based on gas electron multipliers. NIM A628 (2011) 434
Commercial 10×10 cm2 GEM foils, 50 μm /140 μm, from Tech-Etch Corp, Plymouth, MA. 8×8 cm2 sensitive area CCD camera - low noise SBIG ST-6 with thermoelectric Peltier cooling to -30ºC 375×241 pixels, pixel size translates to 0.375×0.375 mm2
at GEM2 location
Sensitive Volume
Alexander Klyachko, IU Cyclotron, PTCOG51, Seoul, South Korea, May 17-19, 2012
Optical readout with Ar/CF4 gas mixture, 5-10% CF4 Optimized for high light yield Somewhat non-tissue-equivalent - underestimation of Bragg peak by ~5%
0 20 40 60 80 100 120 140 160 180 2000.850000000000001
0.900000000000001
0.950000000000001
1
1.05dE/dX, relative to H2O
Air
Ar-5% CF4
CF4
He- 50% CF4
He-4
Proton Energy, MeV
Worth trying He-CF4 gas mixture – stopping power is close to air Emission spectra matches CCD’s quantum efficiency curve Smaller signal – by a factor of ≈3 – but sufficient for dose imaging
Alexander Klyachko, IU Cyclotron, PTCOG51, Seoul, South Korea, May 17-19, 2012
Lateral profiles compared to EBT2 film - good agreement at 50% isodose (within 0.5 mm). Widening of lower part of GEM detector’s profiles is attributed to light reflections in detector
Linear in pulsed beam up to 440 Gy/min Position resolution σ≤0.42 mm (≈pixel size)
Center of SOBP (122 mm water)
Ø20 mmcollimator
-40 -30 -20 -10 0 10 20 30 400
0.2
0.4
0.6
0.8
1 EBT2 film
dGEM
zero depth
Ø20 mmcollim.
zero depthzero depth-30 -20 -10 0 10 20 30
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
EBT2 Film
Ø10 mmcollim.
zero depth
He/CF4 60/40% gas mixture
Alexander Klyachko, IU Cyclotron, PTCOG51, Seoul, South Korea, May 17-19, 2012
Pristine proton field (range 16 cm in water), diameter 50 mm collimator
Alexander Klyachko, IU Cyclotron, PTCOG51, Seoul, South Korea, May 17-19, 2012
Dose ImagingModified Proton Field (Range in Water 16 cm SOBP 4.8 cm)
Ø 20 mm
Ø 10 mm
Ø 20 mm
Ø 10 mm
Alexander Klyachko, IU Cyclotron, PTCOG51, Seoul, South Korea, May 17-19, 2012
Other Applications:
Beam commissioningEspecially scanning systems
Proton radiography
Markus Chamber PinPoint Ion Chamber
10 ms exposure
GEM Image
Alexander Klyachko, IU Cyclotron, PTCOG51, Seoul, South Korea, May 17-19, 2012
Conclusions: We have developed a detector system for two-dimensional dose imaging in
proton therapy based on double-GEM amplification structure. Good linearity in dose rate and energy response. Works in continuous and
scanned beam. Can be made nearly water-equivalent – no underestimation of Bragg peak.
Sub-millimeter position resolution (σ<0.42 mm) and fast response. Both could be improved by using a faster CCD camera with higher pixel count.
Can be used as QA and commissioning detector.
Overall… a promising detector for small field dosimetry. Fabrication of a dedicated small field
detector is underway.
Alexander Klyachko, IU Cyclotron, PTCOG51, Seoul, South Korea, May 17-19, 2012
Thank you