Abstract
BACKGROUND: Ultra-high dose rate radiotherapy (typically defined >40 Gy/s) has shown promise for enhanced normal tissue sparing while maintaining tumor control, termed the FLASH effect when this biological response is observed. However, precise dose rate control remains a technical challenge in proton therapy systems, particularly for research applications investigating dose rate effects. PURPOSE: To develop and validate a passive beam modulation technique using tungsten scatterers for controllable dose rate adjustment in a synchrotron-based proton therapy beamline without modifying accelerator parameters. METHODS: We investigated dose rate modulation using tungsten foils of varying thicknesses (0.1-3.5 mm) positioned upstream in an 87.2 MeV experimental proton beamline. Monte Carlo simulations using Geant4 were performed to model the dose rate variation as a function of the tungsten scatter's thickness. Experimental validation was conducted using an Advanced Markus ionization chamber for dose rate measurements and EBT-XD radiochromic films for lateral dose profile analysis. Five tungsten thicknesses were tested with full-spill deliveries (∼1400 monitor units (MUs), and ∼100 milliseconds (ms) pulse width). RESULTS: An inverse exponential relationship between dose rate and scatterer thickness was observed, with measured dose rates ranging from 288.9±0.7 Gy/s (0.1 mm tungsten) to 10.2±0.2 Gy/s (3.5 mm tungsten). Experimental measurements validated the accuracy of Monte Carlo predictions at the standard condition (1400 MU delivered in 100 ms) by falling within the 95% confidence intervals. Lateral beam profiles demonstrated progressive broadening with increased scatterer thickness, and the film measurements showed good agreements with Monte Carlo simulation (<2% difference at beam center). CONCLUSIONS: Tungsten scatterer thickness modulation provides a practical, controllable method for dose rate adjustment spanning conventional to FLASH regimes. This passive approach enables precise dose rate control for preclinical radiobiological research without requiring modifications of accelerator structure and parameters.