Purpose: To evaluate the sensitivity of different methods of measuring photon beam energy.
Methods: Baseline scans for beam energy of a 6MV beam on an Elekta Versa HD accelerator were obtained using five methods. Depth dose and profile scans at 90cm SSD were measured using a PTW Beamscan 3D scanner with a PTW Semiflex 3D ionization chamber. Measurements were then made using the Daily QA3 and IC Profiler devices from Sun Nuclear, a PTW Farmer chamber in plastic water, and the IQM transmission detector from IRT systems. The beam energy was then adjusted by approximately 0.5% and 1.0% and then back to the baseline energy making a total of 4 sets of measurements.
Results: PDD10 was 66.17, 66.64, 67.05, and 65.96 after return to baseline. This shows that setting the same values on the linac did not return the energy exactly to baseline. This was confirmed by all of the methods. PDD20 was 37.85, 38.48, 39.02, and 37.9. The maximum Off-Axis-Ratio was 1.040, 1.018, 1.005, and 1.039. The energy percentage from the Daily QA3 device was 2.52, 8.99, 15.77, and 2.23. The maximum OAR from the IC Profiler was 1.036, 1.022, 1.009, and 1.035. The energy from the IC Profiler using the energy wedge was 69.74, 70.01, 70.62, and 70.02. The ratio of ionization chamber readings at 10cm and 5cm depth was 0.848, 0.85, 0.852, and 0.847. For 15cm and 5cm depth, it was 0.705, 0.709, 0.714, and 0.703. Using the ratio of the 30x30cm2 and 2x2cm2 field readings for the IQM device the values were 154.0, 150.4. 146.7, and 154.7.
Conclusion: The sensitivity of the 5 methods in detecting the energy change of a 6MV beam was evaluated. There was a wide variation among them and the appropriate tolerances for each method should be set based on these findings.