A comparison of methods for monitoring photon beam energy constancy

In extension of a previous study, we compared several photon beam energy metrics to determine which was the most sensitive to energy change in addition to those, we accounted for both the sensitivity of each metric and the uncertainty in determining that metric for both traditional flattening filter (FF) beams (4, 6, 8, and 10 MV) and for flattening filter-free (FFF) beams (6 and 10 MV) on a Varian TrueBeam. We examined changes in these energy metrics when photon energies were changed to ± 5% and ± 10% from their nominal energies: 1) an attenuation-based metric (the percent depth dose at 10 cm depth, PDD(10)) and, 2) profile-based metrics, including flatness (Flat) and off-axis ratios (OARs) measured on the orthogonal axes or on the diagonals (diagonal normalized flatness, F_DN). Profile-based metrics were measured near d_max and also near 10 cm depth in water (using a 3D scanner) and with ionization chamber array (ICA). PDD(10) was measured only in water. Changes in PDD, OAR, and F_DN were nearly linear to the changes in the bend magnet current (BMI) over the range from -10% to +10% for both FF and FFF beams: a ± 10% change in energy resulted in a ± 1.5% change in PDD(10) for both FF and FFF beams, and changes in OAR and F_DN were > 3.0% for FF beams and > 2.2% for FFF beams. The uncertainty in determining PDD(10) was estimated to be 0.15% and that for OAR and F_DN about 0.07%. This resulted in minimally detectable changes in energy of 2.5% for PDD(10) and 0.5% for OAR and F_DN. We found that the OAR- or FDN- based metrics were the best for detecting energy changes for both FF and FFF beams. The ability of the OAR-based metrics determined with a water scanner to detect energy changes was equivalent to that using an ionization chamber array. We recommend that OAR be measured either on the orthogonal axes or the diagonals, using an ionization chamber array near the depth of maximum dose, as a sensitive and efficient way to confirm stability of photon beam energy.