1. Alexander Klyachko, IU Cyclotron, PTCOG51, Seoul, South Korea, May 17-19, 2012 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. 2 Indiana University Health Proton Therapy Center, Bloomington, Indiana, USA 3 Indiana University School of Medicine, Indianapolis, Indiana, USA

2. 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

3. Alexander Klyachko, IU Cyclotron, PTCOG51, Seoul, South Korea, May 17-19, 2012  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.

4. 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

5. 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  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

6. 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-CF 4 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 cm 2 GEM foils, 50 μm /140 μm, from Tech-Etch Corp, Plymouth, MA.  8×8 cm 2 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 mm 2 at GEM 2 location Sensitive Volume

7. Alexander Klyachko, IU Cyclotron, PTCOG51, Seoul, South Korea, May 17-19, 2012  Optimized for high light yield  Somewhat non-tissue-equivalent - underestimation of Bragg peak by ~5%  Worth trying He-CF 4 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

8. Alexander Klyachko, IU Cyclotron, PTCOG51, Seoul, South Korea, May 17-19, 2012 Center of SOBP (122 mm water) Ø20 mm collimator zero depth Ø20 mm collim. zero depth Ø10 mm collim. zero depth He/CF 4 60/40% gas mixture

9. Alexander Klyachko, IU Cyclotron, PTCOG51, Seoul, South Korea, May 17-19, 2012

10. Ø 20 mm Ø 10 mm Ø 20 mmØ 10 mm

11. Alexander Klyachko, IU Cyclotron, PTCOG51, Seoul, South Korea, May 17-19, 2012 10 ms exposure

12. 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.

13. Alexander Klyachko, IU Cyclotron, PTCOG51, Seoul, South Korea, May 17-19, 2012 Thank you