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

Share this article

Awards Nomination

Editors List

  • Prof. Elhadi Miskeen

    Obstetrics and Gynaecology Faculty of Medicine, University of Bisha, Saudi Arabia

  • Ahmed Hussien Alshewered

    University of Basrah College of Medicine, Iraq

  • Sudhakar Tummala

    Department of Electronics and Communication Engineering SRM University – AP, Andhra Pradesh

     

     

     

  • Alphonse Laya

    Supervisor of Biochemistry Lab and PhD. students of Faculty of Science, Department of Chemistry and Department of Chemis

     

  • Fava Maria Giovanna

     

Onkologia i Radioterapia peer review process verified at publons
Indexed In
  • Directory of Open Access Journals
  • Scimago
  • SCOPUS
  • EBSCO A-Z
  • MIAR
  • Euro Pub
  • Google Scholar
  • Medical Project Poland
  • PUBMED
  • Cancer Index
  • Gdansk University of Technology, Ministry Points 20