Increased flexibility and efficiency of a double-scattering FLASH proton beamline configuration for in-vivo SOBP radiotherapy treatments.

To commission a proton, double-scattering FLASH beamline by maximizing efficiency and field size, enabling higher-LET FLASH radiotherapy of cells and small animals using an SOBP treatment configuration. We further aim to provide a configuration guide for the design of future FLASH proton DS beamlines.

Approach: Beam spot size and spread were measured with film and implemented into TOPAS (TOol for PArticle Simulation). Monte Carlo simulations were optimized to verify the ideal positioning, dimensions, and material of scattering foils, secondary scatterers, ridge filters, range compensators, and apertures. A ridge filter with three discrete heights was used to create a spread-out Bragg peak (SOBP) and was experimentally verified using our in-house experimental FLASH beamline. The increase in dose rate was compared to nominal shoot-through techniques. 

Results: The configuration and scatterer distance producing the largest field size of acceptable flatness, without drastically compromising dose rate was determined to be an elliptical field of 2 cm x 1.5 cm (25% larger than a previous configuration). SOBP testing yielded three distinct but connected spikes in dose with flatness under 5%. Reducing the thickness of the (first) scattering foil by a factor of two was found to increase efficiency by 50%. The new settings increased the field size, provided a Bragg peak treatment option, and increased the maximum available dose rate by 85%, as compared to the previous, shoot through method. 

Significance: beam line updates established FLASH dose rates of over 135 Gy/s (potentially higher) at our double-scattering beamline, increased the efficiency and field size, and enabled SOBP treatments by incorporating an optimized ridge filter. Based on our simulations we provide parametric suggestions when commissioning a new proton DS beamline. This enhanced FLASH beamline for SOBP irradiations with higher dose rates and larger field sizes will enable a wider variety of experimentation in future studies.