WE‐E‐141‐01: Accurate Calibration and Measurement of Unflattened Photon Beams

Purpose: To accurately quantify and account for systematic errors in the measurement of dose for unflattened photon beams due to characteristics specific to these beams. Methods: Dose measurements were taken on five photon beams (6X, 6X FFF, 10X, 10X FFF, and 15X) on a Varian TrueBeam STx using one diode and five ion chambers (PTW Farmer, CC13, CC04, and PTW 3D Pinpoint) which were selected to span a range of dimensions. Errors in the percent depth dose (PDD) due to depth dependent P_ion effects were measured. High‐resolution diode scans for profiles at 6X FFF and 10X FFF were used to create an analytical fit model. This model was used to calculate the reduction in dose (caused by volume averaging) in unflattened beams due to chamber size. Each chamber was cross‐calibrated (using the flattened beams) against the farmer chamber to determine N_D,w(Co‐60). These N_D,w(Co‐60) values were then used to determine the absolute dose in the FFF beams. Results: Changes in P_ion were most pronounced in the 10X FFF beam. This effect resulted in a 1% and 1.5% increase in the measured PDD at depths of 10cm and 20cm, respectively, for 10X FFF with the CC13 chamber. The farmer chamber had the most significant volume averaging effect with a calculated (and measured) dose reduction of 0.35% and 0.72% for 6X FFF and 10X FFF, respectively. An analytical correction factor, P_Flat, was developed to account for this effect. Conclusion: Corrections are necessary to reduce the errors produced when measuring dose in unflattened beams. Two types of errors were quantified that will each lower the measured dose to dmax. When following a TG‐51 calibration, uncorrected absolute dose measurements may Result in a worst‐case error on the order of 2%. We developed an analytical correction factor that substantially reduces this error for common farmer chambers.