Assessment of ion recombination and polarization correction factors for small and large field size of photon beams

Abstract

Author(s): M. Boopathi, D. Khanna*, P. Mohandass, P. Venkatraman and V. Gogul Priean

Purpose: To investigate ion recombination (Ks ) and polarity correction factor (Kpol) for small and large field sizes using small volume ionization chamber for Flattening Filter (FF) and Flattening Filter Free (FFF) beams of Varian TrueBeam STx linear accelerator. Materials and Methods: All the readings were measured on PTW BEAMSCAN® water phantom at 100 cm Source to Surface Distance (SSD) at dmax and 10cm depth for 6, 10, 15, 6FFF and 10FFF mega voltage photon beams with maximum dose rate for square fields from 0.5 cm²×0.5 cm² to 30 cm²×30 cm² . Two ion chambers such as PTW Semiflex 3D 31021 and Farmer chamber 30013 of volumes 0.07 cc and 0.6cc respectively were hired. The correction factors were computed from the readings according to the protocol no 398 of International Atomic Energy Agency’s Technical Report Series (IAEA TRS 398). The Ion recombination values obtained from “Two-Voltage Method” (TVM) were verified with 1/V versus 1/Q curves (Jaffé-plots) for all the beam energies. Results: From the result, the Ion recombination correction factors (Ks ) never exceeded 1.032, additionally the Jaffé-plot's results agree very well with TVM values (varies up to 0.3%), except for square fields 0.5 cm²×0.5 cm² and 1 cm²×1 cm² (up to 8%). The Ks values are completely independent of field sizes for all beam energies. The Kpol values varies independently with field sizes up to a square field 2 cm2×2cm2 , between square fields 2 cm²×2 cm² to 10 cm²×10 cm² the plot shows almost a straight line for all radiation condition. For all the square fields (except 0.5 cm²×0.5 cm² and 1 cm²×1 cm² ), Ks and Kpol values of FFF beams only varies by a maximum of 0.6% and 0.1% from the values of FF beams respectively. Conclusion: The saturation voltage of the small field dosimeters is greater than the dosimeter working voltage. The Ks and Kpol values of small fields are different from standard field (reference field). The ion recombination can be adequately accounted for high dose rate FFF beams using Ks determined with the standard “Two-Voltage Method”. The result obtained from FFF beams doesn’t deviate significantly from flattened beams. The inappropriate readings of square fields 0.5 cm²×0.5 cm² and 1.0 cm²×1.0 cm² may be, also due to the lack of dosimeter response as a result of lack of lateral charged particle equilibrium and volume averaging effect of the chamber

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