Author: Ronald C Chen, Hao Gao, Nicolas Gerard, Harold Li, Yuting Lin, Jarrick Nys, Jufri Setianegara, Mark Szczepanski, Aoxiang Wang 👨🔬
Affiliation: 4Physikalisch-Technische Werkstätten (PTW), Department of Biomedical Engineering, Huazhong University of Science and Technology, Ion Beam Applications (IBA), Department of Radiation Oncology, University of Kansas Medical Center 🌍
Purpose: Modern synchrocyclotrons can produce magnetically-scanned protons at ultra-high dose-rates (FLASH). However, their pulsed nature leads to high doses-per-pulse (DPP) which can cause non-negligible charge recombinations or saturations for ionization chambers (IC) and solid-state detectors respectively. In this work, we investigated the suitability of various commercially-available and prototype detectors (flashDiamond) for FLASH dosimetry within a pulsed proton beam.
Methods: An IBA Proteus®ONE synchrocyclotron was tuned to produce 228MeV FLASH protons at Gantry 0°. All detectors (Razor, Razor Nano, Razor Diode, microDiamond, flashDiamond) were cross-calibrated (PPC05, ) at conventional dose-rates (CONV). Additional CONV characterizations (linearity, dose-rate, energy, PDD, LETd) were performed for flashDiamond prototypes. Absolute FLASH dosimetry was performed using a 3x3cm2 FLASH beam (7x7 spots, 0.5cm spacing) at 2.066cm(WET) depth. FLASH Pion and Ppol were determined for all ICs with Pion calculated using both TRS-398 and Niatel methods. FLASH small-field dosimetry (1.5x-3x3cm2) was performed for appropriately-sized dosimeters.
Results: FLASH nozzle current was measured at 129.5nA with 0.80% s.t.d. and 32.6±0.5cGy DPP. Ppol was similar between CONV and FLASH. FLASH Pion was universally higher (1.0097-1.0294) with similar values between Niatel and TRS-398 approaches. Volumetric recombinations within PPC05 and Razor ICs were detected despite the pulsed beam structure. All dosimeters responded within ±1% of PPC05 measurements except for diodes (3.79% over-response). flashDiamond had negligible finite-volume effects but were relatively insensitive (613.29cGy/nC). Razor Nano and flashDiamond had a ±1% small-field FLASH response as film dosimetry (EBT-XD).
Conclusion:
PPC05 and Razor chambers exhibited unexplained volumetric recombinations in pulsed proton FLASH beams. Niatel and TRS-398 approaches were still valid for small FLASH Pion values under 1.03. Except for diodes, all detectors agreed within ±1% of one another and can be used for accurate absolute proton FLASH dosimetry. Razor Nano and flashDiamond are appropriate for accurate small-field FLASH dosimetry.