Comparison of derived correction factors for effects of ion recombination and photon beam quality index following TG-51 and TRS-398 dosimetry protocols.

In this study, ion recombination correction factor (k) and beam quality conversion factor ( [Formula: see text] ) values were extracted following the recommendations of the TRS-398 and TG-51 dosimetry protocols for widely used cylindrical ionization chambers for high energy photon beam dosimetry to quantify the agreement between the instructions for these two protocols for absolute dosimetry inside water. Four different types of cylindrical ionization chambers comprising Farmer (TM30013), Semiflex 0.125 cm (TM31010), Semiflex 0.3 cm (TM31013), and PinPoint (TM31016) were considered, and k and [Formula: see text] values were determined at photon energies of 6 MV and 15 MV. The maximum difference between the measured k values according to the instructions in the TRS-398 and TG-51 protocols was 0.03%. The k data measured with both protocols agreed well with those measured by using the Jaffe-plot approach, where the maximum difference was about 0.33%. The observed differences between the [Formula: see text] factors measured by using the TRS-398 and TG-51 dosimetry protocols at photon energies of 6 MV and 15 MV were 0.37% and 0.55%, respectively. The [Formula: see text] values measured using the TG-51 dosimetry protocols were slightly closer to those measured by a reference ionization chamber dosimeter. We conclude that the maximum differences were about 0.4% and 0.6% in the absorbed dose measurements according to the TRS-398 and TG-51 instructions at photon energies of 6 MV and 15 MV, respectively. The type of ionization chamber employed also affected the differences, where the maximum and minimum dose differences were found using the Farmer and PinPoint chambers, respectively.