Author: Winter Green, Yao Hao, Nels C. Knutson, Matthew C. Schmidt, Xiandong Zhao 👨🔬
Affiliation: Washington University School of Medicine in St. Louis, Washington University School of Medicine, Department of Radiation Oncology, Washington University School of Medicine, WashU Medicine 🌍
Purpose: Beam profile flatness is crucial for both photon and proton radiotherapy systems. This work investigated the dosimetry impact of beam flatness on a clinical synchrocyclotron proton system.
Methods: The timing of the dosimetry system was detuned to produce unflat beam profile. Open fields (10cm x 10cm) were delivered for five different energies: full energy 227MeV, 200MeV, 150MeV, 100MeV, and 50MeV. In addition, two clinical plans, with energies ranging from 133.11MeV to 189.85MeV, were delivered. MatriXX 2D array device measurements were collected for detuned timing and correct timing. Gamma analyses were performed for both scenarios.
Results: Compared to correct timing, the maximum increased flatness of detuned timing deliveries for all five open filed energies from high to low are: 3.90%, 2.07%, 1.54%, 0.79%, and 0.31%. Using clinical plans as references, the maximum 3%&3mm and 2%&2mm 2D gamma difference between correct and detuned timings are 0.9% and 2.5%, respectively. All measurements would be considered passing in a clinical scenario. In the direct comparison between correct and detuned timings, the full energy 227MeV plan shows the lowest 3%&3mm and 2%&2mm passing rates: 93.9% and 85.8%, respectively.
Conclusion: High-energy open fields are more sensitive to beam unflatness. The gamma rate of the clinical plan with mixed energies is less sensitive to the detuning in this study. Suggested periodic open-field measurements may assist in catching flatness issues on proton accelerator systems.