Author: Muhammad Ramish Ashraf, Dixin Chen, Aaron Garza, Lloyd Emmanuel Kamole Ghomsi, Edward Elliot Graves, Billy W Loo, Stavros Melemenidis, Joseph B. Schulz, Lawrie Skinner, Murat Surucu 👨🔬
Affiliation: Department of Radiation Oncology, Department of Radiation Oncology, Stanford University School of Medicine 🌍
Purpose: FLASH radiotherapy, supported by robust preclinical evidence showing an improved therapeutic index, is advancing toward clinical trials. Clinical FLASH treatments will likely require scanning delivery to cover typical target volumes across all beam types including protons and photons. While scanning may extend the total treatment time beyond the canonical FLASH regime, the local irradiation time can remain short. This simulation study aims to dosimetrically characterize a slit scanning platform designed to emulate clinical FLASH radiotherapy and support in vivo experiments that model the biological effects of temporal delivery patterns.
Methods: A linac-based 14MeV electron FLASH beam was modeled in TOPAS. A tungsten collimator with slit widths ranging from 1mm to 4mm was placed along the beamline, followed by a water phantom mounted on a robot arm. The robot arm moved the phantom across the slit in a step-and-shoot manner, synchronized to the pulsed beam. Irradiation over a 1.2-cm wide field was simulated as a sweep scan with 10 pulses delivered per slit-width step. Effective dose rates and beam characteristics were evaluated for each slit width and compared to static irradiation over the scanned region.
Results: Slit widths of 3mm and 4mm achieved ultra-high surface dose rates with minimum stepping speeds of 65mm/s and 44mm/s respectively. At a stepping speed of 150mm/s, the 4mm slit maintained a mean local dose rate of (72±17)Gy/s at the surface and (53±9)Gy/s at a depth of 1.9cm upon exiting the phantom. Dose profiles of the scanned surface were comparable to the static irradiation profile, with flatness and penumbra differences within 5% and 0.3mm across all slit widths.
Conclusion: This simulation demonstrates that slit scanning can deliver ultra-high dose rates while maintaining dose profiles comparable to static irradiation. We will follow up with experiments using our developed robot arm to validate the simulation results.