Author: Davide Brivio, Lia Carroll, Juliane Daartz, Jan PO Schuemann, Piotr Zygmanski 👨🔬
Affiliation: Massachusetts General Hospital, Department of Radiation Oncology, Brigham and Women’s Hospital, Dana-Farber Cancer Institute, Harvard Medical School, Massachusetts General Hospital and Harvard Medical School 🌍
Purpose: We employ High-Energy Current (HEC) detectors for real-time monitoring of proton beams at conventional to FLASH dose rates. This would help overcome the need for fast and accurate dosimeters in FLASH proton therapy and provide a new tool in conventional proton beams.
Methods: Secondary electrons produced by proton beams induce an electrical current in an array of thin film electrode arrays producing an output current proportional to dose rate. We built such a detector using 70 um of aluminum, 50 um of tantalum, and 50 um aerogel arranged like Al-PA-Ta-PA-Al (aluminum-aerogel-tantalum-aerogel-aluminum).
A cyclotron-produced scattered 228 MeV proton beam was used to irradiate the detector. Beam output was characterized using an exradin T1 chamber. Dose linearity was measured from 67-6700 Gy with a Kethley electrometer. Further irradiations were performed with a collimated beam (1.7 cm X 1.3 cm oval), and a digitizer was used to monitor the detector response. Dose rate response was evaluated by measurement detector current with dose rates varying from 9 - 225 Gy/s with 40 Gy irradiations and from 2.5 – 115 Gy/s with 10 Gy irradiations.
Results: Detector response was linear with dose (Charge = 1.80*10-3 nC/Gy, R2=0.99). The detector charge was flat with dose rate, and the detector current was linear with dose rate (40 Gy: Current =7.29*10-4 nA/(Gy/s), R2=0.99) (10 Gy: Current =1.21*10-3 nA/(Gy/s), R2=0.99). SNR for current measurements varied from 16-274 for 40 Gy irradiations and 1.5 – 100 for 10 Gy irradiations.
Conclusion: The HEC detector shows dose linearity up to 6700 Gy at FLASH dose rates and dose-rate independence from 2.5-225 Gy/s. An optimized electronic readout shows a 100x gain in SNR compared to what was used for these measurements. This would allow us to measure dose rates as low as 0.1 Gy/s.