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Small-Field Studies in Proton Therapy Beams with a GEM-based Dose Imaging Detector

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.

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Small-Field Studies in Proton Therapy Beams with a GEM-based Dose Imaging Detector

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  1. A.V. Klyachko1, D.F. Nichiporov1, L. Coutinho2,C.-W. Cheng2, 3, M. Luxnat1, I. J. Das2, 3 1Indiana 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 Small-Field Studies in Proton Therapy Beams with a GEM-based Dose Imaging Detector

  2. Outline: Why small fields? Why GEM detector? GEMs in dose imaging – basic principles, optical readout, detector design Test results Summary

  3. 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.

  4. WET=0 8 cm 15 cm Beam range 16 cm in water 15.8 cm

  5. Why GEMs: Existing detectors used in clinical practice all have notable shortcomings when applied to small field dosimetry: • Nonlinear dose and energy response • Long measuring time for obtaining complete 2D dose distributions • Insufficient spatial resolution • Tissue non-equivalence • Or a combination thereof Gas Electron Multipliers = GEMs (Sauli 1997) show promise to be free of those drawbacks • fast performance • robustness and design flexibility • excellent spatial resolution • cascade option to improve signal-to-noise ratio • electronic and optical readout • schemes

  6. Optical Readout of GEMs Sensitive Volume • 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 J.H. Timmeret al, A scintillating GEM for 2D-dosimetry in radiation therapy. NIM A478 (2002) 98 F.A.F. Fragaet 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. Seravalliet 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

  7. 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% • 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

  8. He/CF4 60/40% gas mixture Linear in pulsed beam up to 440 Gy/min Position resolution σ≤0.42 mm (≈pixel size) Ø20 mm collim. Ø10 mm collim. Ø20 mm collimator Center of SOBP (122 mm water) zero depth zero depth zero depth zero depth 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

  9. Pristine proton field (range 16 cm in water), diameter 50 mm collimator

  10. Dose Imaging Modified Proton Field (Range in Water 16 cm SOBP 4.8 cm) Ø 20 mm Ø 20 mm Ø 10 mm Ø 10 mm

  11. Other Applications: Proton radiography Markus Chamber GEM Image Beam commissioning Especially scanning systems PinPoint Ion Chamber 10 ms exposure

  12. Conclusions: Overall… a promising detector for small field dosimetry. Fabrication of a dedicated small field detector is underway. 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.

  13. Thankyou

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