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

Area of Science:

• Medical Physics
• Radiation Oncology
• Accelerator Technology

Background:

• Ultra-high dose rate (UHDR) radiotherapy, also known as FLASH-RT (>40 Gy/s), shows reduced normal tissue toxicity while maintaining tumor control compared to conventional dose rates.
• Implementing UHDR requires significant linear accelerator (LINAC) modifications, which can be challenging without dedicated engineering support.
• Existing upstream scatter foil methods struggle to achieve useful field sizes at UHDR.

Purpose of the Study:

• To present a novel configuration for achieving UHDR using downstream electron scattering foils.
• To enable FLASH-RT at the isocenter by optimizing foil position and thickness.
• To characterize achievable dose rates and dose uniformity for preclinical applications.

Main Methods:

• A decommissioned Varian CLINAC 21EX linear accelerator was modified for UHDR by using 16 MV beam settings, removing the x-ray target and flattening filter.
• Downstream beam scattering was evaluated using four lead foil thicknesses (0.28-1.2 mm) at three source-to-foil distances (75.1-90.0 cm).
• Beam profiles were measured using Gafchromic EBT-XD films at isocenter, and PDDs were measured to assess Bremsstrahlung contribution.

Main Results:

• Without scattering, peak dose rates exceeded 200 Gy/s but had poor uniformity (flatness = 22.0%).
• Standard dual-foil systems achieved good uniformity (flatness = 6.1%) but not UHDR (10-11 Gy/s).
• Downstream scattering achieved UHDR (43-135 Gy/s) with improved beam uniformity (flatness 1.2%-3.5% at 75.1 cm source-to-foil distance). Thicker foils reduced practical range and increased Bremsstrahlung.

Conclusions:

• Downstream electron scattering is a practical method for achieving UHDR (43-135 Gy/s).
• This approach maintains acceptable beam uniformity near the isocenter.
• Enables FLASH-RT for preclinical studies using modified LINACs.