Purpose: To optimize, build and commission a novel Bremsstrahlung target and megavoltage irradiation platform for delivering ultrahigh dose-rate radiation to small-animals in an exploration of X-ray FLASH radiotherapy at TRIUMF.
Methods: The design of a tantalum-aluminum (Ta-Al) explosion-bonded conversion target has been optimized using Monte Carlo (MC) and Finite Element Analysis (FEA) simulation methods. The water-cooled Ta-Al target facilitates conversion of a 10MeV, 1kW electron beam at the ARIEL e-linac into an ultrahigh dose-rate (>40Gy/s) x-ray source with <1s pulse-lengths and 0.05%-100% Duty Factor (FLASH mode) while preserving routine beam-dump capabilities in a lower power steady-state. Dose rates in water-phantoms were calculated in slab and CAD geometries using EGSnrc and TOPAS MC codes, respectively, to inform the design of the Ta-target, Al-flange and W-shield thicknesses and geometry. Thermo-mechanical FEA simulations in ANSYS subsequently informed the stress-strain conditions and fatigue life of the target assembly under prescribed conditions resulting in the re-design, and thus revised dose-rate capabilities, of the final prototype. Erosion tests and commissioning will be followed by dosimetric characterization ahead of FLASH treatment planning in small-animal models.
Results: Simulated water-phantom irradiations demonstrate that surface dose-rates of 128Gy/s may be achieved for a 1x1cm² field size and 7.5cm source-to-surface distance using the treatment-beam configuration (E=10MeV,2σ=5mm,P=1kW). Dose rates >40Gy/s are maintained to a depth of 5cm in water. Modular collimation and SSD flexibility allows for >200Gy/s to be achieved at larger field sizes. Assembly temperatures are maintained below the Ta, Al and cooling-water thresholds of 2000, 300 and 100°C, respectively, while the Al strain behavior remains everywhere elastic.
Conclusion: The ARIEL x-ray FLASH experimental platform achieves ultra-high dose-rates within the thermo-mechanical constraints of the system. Target cooling, mechanical robustness and failure mitigation have culminated in a design intended for FLASH and steady-state applications with installation and commissioning planned for May 2021.
Funding Support, Disclosures, and Conflict of Interest: This work was partially funded by the National Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant, Canada Research Chair program and the New Frontiers in Research Fund (NFRFE)