FAST-X: Design and Optimization of a Focused Multi-Source X-Ray Platform for Ultra-High Dose-Rate Radiation Delivery
Abstract
Purpose
To develop FAST-X to deliver ultra-high dose-rate (UHDR) irradiation for complex cranial targets.
Methods
FAST-X consists of 21 X-ray sources operated at a tube voltage of 600kV. A bending magnet steers the electron beam to enable a transition from coplanar to conical beam arrangements, and a scanning magnet moves resulting X-ray beamlets to cover large targets, using beamlet widths of 5, 10, 15, and 20mm. Monte Carlo (MC) simulations using TOPAS/TOpasOpt optimized beamline geometry, including target type (reflective/transmission), different filtrations (Cu- Al-Be, Mo, or diamond), collimation, and source-to-isocenter distance, to maximize dose-rate while minimizing electron contamination, leakage, and beam penumbra at 75mm depth in water. The thermal feasibility of optimized target-filtration designs was evaluated using COMSOL Multiphysics. An in-house treatment-planning-system (FAST-X-TPS) was developed to jointly optimize absorbed dose and local-dose-rate under target/organ-at-risk (OAR) constraints. TPS dose was evaluated using independent MC recalculation and compared with a reference Gamma-Knife (GK) plan for an intracranial metastatic case with four small-volume targets treated with 35Gy in 5-fractions.
Results
Optimal FAST-X target-filtration designs consisted of a 50µm tungsten target with Cu-Al-Be filtration (4.42-0.94-1.26mm), diamond filtration (443µm), or Mo filtration (123µm). The optimized transmission target design nearly doubled the dose rate compared to a reflection target. Maximum sustainable current of approximately 15mA/source (W-Cu-Al-Be), 13mA/source (W-Mo), and 40mA/source (W-diamond) was calculated. For a 15mm beamlet, W-diamond increased dose rate by ~430% versus W-Cu-Al-Be (W-Mo:~66%), while maintaining <0.5% electron contamination and a penumbra ~1.7mm. The FAST-X plan achieved conformity comparable to GK plan with an irradiation time of ~850ms, while delivering a mean local-dose-rate of ~10.25Gy/s in GTVs and a maximum local-dose-rate of ~2Gy/s in OARs. MC recalculation showed agreement with TPS.
Conclusion
FAST-X demonstrated feasibility for UHDR radiation delivery through MC optimization, thermal modeling, and dose-rate-based treatment planning, while maintaining dose conformity for cranial targets.