A preclinical setup for FLASH radiotherapy beam delivery at isocenter using downstream electron scattering

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BACKGROUND: Preclinical investigations have shown that ultra-high dose rate (UHDR) radiotherapy (>40 Gy/s) also known as FLASH-RT, reduces normal tissue toxicity while preserving tumor control efficacy compared to conventional dose rate treatments. Achieving a UHDR-capable LINAC requires modifications including modifications to electron gun current and bending magnet configurations, use of dedicated energy control boards and use of lower energy scatter foils. However, in the absence of engineering support to implement these changes, it is challenging to realize useful field sizes at ultra-high dose rates using the upstream scatter foils alone. PURPOSE: In this study, we present a configuration to achieve UHDR utilizing downstream electron scattering foils to enable FLASH-RT at isocenter. By changing the scattering foil position and thickness, we aim to characterize achievable dose rates and dose uniformity for preclinical studies. METHODS: A decommissioned Varian CLINAC 21EX linear accelerator was modified for UHDR irradiation by using the 16 MV beam settings while removing the x-ray target and flattening filter. Downstream beam scattering was evaluated by testing four lead foil thicknesses (0.28-1.2 mm) at three different source-to-foil distances (75.1-90.0 cm), with beam profiles measured using Gafchromic EBT-XD films at isocenter and evaluated using flatness metrics. The PDDs were also measured to characterize the impact of foils on Bremsstrahlung contribution. RESULTS: Without a scattering foil, peak dose rates exceeded 200 Gy/s but with poor uniformity (flatness = 22.0%). Standard dual-foil systems achieved good uniformity (flatness = 6.1%) but yielded non-UHDR (10-11 Gy/s). With downstream scattering, UHDR (43-135 Gy/s) with improved beam uniformity was achieved. At 75.1 cm source to foil distance, Dmax flatness values at isocenter ranged from 3.5% to 1.2% with increasing foil thickness. However, thicker foils resulted in reduced practical range and increased Bremsstrahlung contribution. CONCLUSION: This study demonstrates that downstream electron scattering provides a practical approach for achieving UHDR (43-135 Gy/s) while maintaining acceptable beam uniformity close to the isocenter.

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