Author: Dale W. Litzenberg, Ikechi S Ozoemelam, Rowan Paplanus, Michael Pillainayagam, Donald A. Roberts 👨🔬
Affiliation: University of Michigan, Oregon Health & Science University 🌍
Purpose: Clinical linear accelerators use dual foil scattering systems designed to achieve large uniform fields (up to 25×25cm²), which can reduce beam fluence by up to 93% compared to the unmodified beam. This significant reduction in fluence has led to FLASH modifications focused on increasing electron gun current, adjusting bending magnet settings, or operating at reduced source-to-surface distances to exploit the inverse square law. This study evaluates a downstream electron scattering approach to achieve FLASH dose rates while maintaining acceptable beam uniformity at isocenter with minimal accelerator modifications.
Methods: A Varian Clinac 21EX was modified to deliver 16MeV electron beams by operating in 16MV mode with the x-ray target and flattening filter removed. Beam delivery was controlled by monitoring the RF waveform and gating the electron gun firing. Downstream electron scattering used lead foils (0.28-1.2mm) positioned at 75.1-90cm source-to-surface distances. Measurements compared standard dual-foil upstream scattering systems with and without secondary foils. Beam characterization used Gafchromic EBT-XD film dosimetry to measure dose rates, field uniformity, and depth dose curves.
Results: Without scattering foil, peak dose rates exceeded 200Gy/s but with poor uniformity (flatness=22.0%). Standard dual-foil systems achieved good uniformity (flatness=6.1%) but yielded non-FLASH dose rates (10-11 Gy/s). Downstream scattering maintained FLASH dose rates while improving beam uniformity. At 75.1cm SSD, flatness values ranged from 3.5% to 1.2% with increasing foil thickness. However, thicker foils resulted in reduced practical range and increased bremsstrahlung contribution.
Conclusion: The use of a downstream electron scattering configuration allows achieving uniform dose distribution at clinically relevant distances, addressing critical challenges in experimental FLASH radiotherapy research. This approach lowers the barrier to entry for FLASH radiation therapy research and clinical translation.