Detector comparison for small field output factor measurements with flattening filter free photon beams 1 Wolfgang Lechner, 1 Lukas Sölkner, 2 Paulina Grochowska, 1 Dietmar Georg 1 Christian Doppler Laboratory for Medical Radiation Research for Radiation Oncology, Medical University of Vienna, Austria 2 International Atomic Energy Agency, A-1400 Vienna, Austria
Acknowledgements Visit http://www.meduniwien.ac.at/hp/radonc/ Hugo Palmans, Peter Sharpe, Rebecca Nutbrown, Clare Gouldstone, Russel Thomas, Martin Kelly, Joanna Izewska, Godfrey Azangwe, The financial support by the Federal Ministry of Economy, Family and Youth and the National Foundation for Research, Technology and Development is gratefully acknowledged. 2
Motivation Properties of FFF-beams: Higher dose rate, reduced scatter FFF Small fields dosimetric errors are considerably larger Formalism for reference dosimetry of small and nonstandard fields Alfonso et al. (2009) Med Phys 35(11) Which detectors are suitable? Are the detector correction factors of small fields output factor measurements different for FF- and FFF-beams? 3
Setup for output factor (OF) measurements Elekta Precise Linac 6MV and 10MV FF- and FFF-beams BrainLab M3 µmlc for field shaping Field sizes: 10x10cm² to 0.6x0.6cm² 5 measurements per field Electrometer: PTW Unidos Webline Bragg peak chamber as reference 6MV FFF 6MV FF 10MV FFF 10MV FF TPR 20/10 0.684 0.686 0.714 0.735 4
Detectors 14 detectors classified according to their active volume Volume averaging correction factors were calculated for each detector Micro Mini Macro V < 10 mm³ 10 mm³ V 40 mm³ V > 40 mm³ PTW microdiamond IBA CC01 PTW Semiflex 0.125 IBA SFD PTW PinPoint14 IBA CC13 PTW Diode P PTW PinPoint16 IBA IC10 IBA EFD IBA CC04 NPL 2611 IBA PFD PTW microlion Beam profiles were acquired prior to measurements detector was positioned at the center of the beam 5
Detectors Reference dosimeter: Alanine in solid water phantom, same geometry Field width Alanine 5 mm Alanine 2.5 mm 1.8 cm X 1.2 cm X X 0.6 cm X Evaluation: Output factors (OF) were normalized to 3x3 cm² The ratio of the OF measured with detectors to OF measured with alanine was evaluated S S cp cp (detector (alanine ) ) 6
Qualitative results Solid state detectors tended to overestimate the small field output factors Ionization chambers tended to underestimate the small field output factors Similar behavior for FF- and FFF-beams 7
Results ionization chambers CC01 agreed within2% with alanine; tended to over-respond for larger field sizes PinPoint 16 underestimated OF by up to 9%; agreed well for larger field sizes No substantial difference between FF- and FFF-beams 8
Results shielded diodes Substantial difference between 10MV FF- and FFF- beams at 0.6x0.6cm² 9
Results shielded diodes Substantial difference between 10MV FF- and FFF- beams at 0.6x0.6cm² Effect not present for 6MV 10
Results Volume averaging correction factor No correction was necessary for field widths larger than 1.8cm Field width Micro Mini Macro 0.6 cm 1 1.011 1.018-1.029 1.064-1.204 Uncertainties (1SD) major component was the positioning uncertainty Field width Micro Mini Macro 0.6 cm 1% 1.2% 2% 11
Summary & Conclusion The qualitative behavior of detectors is similar for FF- and FFF-beams Substantial difference between FF- and FFF-beams was only observed for shielded diodes irradiated with 10 MV There might be a difference between correction factors for FF- and FFFbeams for other detectors, but it could not be resolved in a clinical environment Macro detectors should not be used for small field dosimetry Thanks for your attention! 12