Comparison of the recommendations of the AAPM TG-51 and TG-51 addendum reference dosimetry protocols

This work quantified differences between recommendations of the TG-51 and TG-51 addendum reference dosimetry protocols. Reference dosimetry was performed for flattened photon beams with nominal energies of 6, 10, 15, and 23 MV, as well as flattening-filter free (FFF) beam energies of 6 and 10 MV, following the recommendations of both the TG-51 and TG-51 addendum protocols using both a Farmer (R) ionization chamber and a scanning ionization chamber with calibration coefficients traceable to absorbed dose-to-water (D-w) standards. Differences in D-w determined by the two protocols were 0.1%-0.3% for beam energies with a flattening filter, and up to 0.2% and 0.8% for FFF beams measured with the scanning and Farmer (R) ionization chambers, respectively, due to k(Q) determination, volume-averaging correction, and collimator jaw setting. Combined uncertainty was between 0.91% and 1.2% (k = 1), varying by protocol and detector.