Determination of the Effective Point of Measurement for Ion Chambers in Electron Beams

The American Association of Physicists in Medicine (AAPM) Task Group (TG) report 51 is the current protocol for calibration of linear accelerator output. TG51 relies on the use of effective point of measurement (EPOM) shifts for ionization chambers to convert from a relative charge reading to a relative dose. For electron beams, TG51 recommends generic EPOM shifts of 0.6 times the cavity radius and no shift for cylindrical and plane-parallel ionization chambers, respectively. Recent literature has shown that using generic instead of chamber-specific EPOM shifts can result in miscalculations of relative dose profiles, beam quality correction factors, and machine output. Therefore, it is the purpose of this work to determine chamber-specific EPOM shifts in electron beams using organic scintillation detectors.