SU‐F‐T‐488: Comparison of the TG‐51 and TG‐51 Addendum Calibration Protocols

Abstract

Purpose:To quantify differences between the TG51 and TG51 addendum calibration protocols.Methods:Beam energies of 6X, 6XSRS, 10X, 15X, 23X, 6XFFF, and 10XFFF were calibrated following both the TG51 and TG51 addendum protocols using both a Farmer and a scanning ionization chamber with traceable absorbed dose‐to‐water calibrations. For the TG51 addendum procedure, the collimating jaws were positioned to define a 10×10cm2 radiation field, a lead foil was only used for kQ measurements of FFF energies, and a volume‐averaging correction was applied based on crossline and inline dose profiles. For the TG51 procedure, the collimating jaws were set to 10×10cm2 according to the digital readout, and a lead foil was used for kQ measurements of energies greater than 10MV.Results:For beam energies with a flattening filter, absorbed dose‐to‐water determined by the two protocols differed by 0.1%–0.3%. For FFF beam energies, differences between the protocols were up to 0.2% and 0.8% for the scanning and Farmer ionization chambers, respectively. Differences between the protocols were due to kQ determination, volume‐averaging correction, and measurement of raw ionization. Differences in kQ values between the two protocols were up to 0.4% and 0.2% for the scanning and Farmer ionization chambers, respectively. Volume‐averaging corrections were less than 0.1% for the scanning ionization chamber, and up to 0.4% and 0.6% for the Farmer ionization chamber in beams with a flattening filter and FFF beams, respectively. Raw ionization measurements differed up to 0.3%±0.07% due to differences in jaw settings.Conclusion:The TG51 and TG51 addendum calibration protocols differed less than 0.3% for the scanning ionization chamber. For the Farmer chamber in FFF energies, volume‐averaging corrections of up to 0.6% contributed to calibration differences of up to 0.8%. Failure to verify the radiation field size can produce calibration differences of up to 0.3%.