Author: Sean L. Berry, Hsiang-Chi Kuo, Seng Boh Gary Lim, Ingrid Valencia Lozano, Laszlo Voros 👨🔬
Affiliation: Department of Medical Physics, Memorial Sloan Kettering Cancer Center 🌍
Purpose:
To quantify the impact of target degradation on 6FFF beam output by analyzing the output trends and adjustment frequencies at our clinic on a flattening filter-free (6FFF) and a flattened (6MV) beam. Longitudinal beam characteristics were evaluated including the time during the chamber and target replacement services.
Methods:
Daily output data for both 6FFF and 6MV were acquired with the same Daily QA3 (SunNuclear Melbourne, FL) device since the installation of the linac. Analysis was done on the unadjusted output curves which were derived by applying the magnitude of the output adjustment after monthly QA to the daily output records. The beam characteristics were compared against reference data to evaluate the impact of target degradation.
Results:
To ensure that the output remained within 2% as per institutional and professional society guidelines the 6FFF and 6MV beam outputs were adjusted approximately twice a year for the first 3.5 years. The 6FFF output started to show an unusual downward drift at 3.5 years, and the frequency of 6FFF output adjustments increased to every 2-3 months until the target was replaced. Observed output change for 6FFF vs. 6MV beams was downwards 16% vs. upwards 8% over the last 30 months. An immediate output measurement after the target replacement showed a 20% output increase at 6FFF while there was no output difference for other beams. Besides the behavior of downward output at 6FFF the beam energy, flatness and symmetry were not impacted by the target degradation.
Conclusion:
Despite sharing the same target and beam spot, an FFF beam has a different output trend than a flattened beam. The separation of the respective output trend lines reveals the target's degradation. Under routine QA, target degradation does not impact significant beam characteristics. However, when target deterioration becomes serious, FFF beam output decreases faster.