Purpose: Clinical use of ultra-high dose-rate (UHDR, >40Gy/s) electron FLASH radiotherapy (eFLASH-RT), a modality demonstrating greater normal tissue sparing than conventional radiotherapy with equivalent tumor control, is constrained by the availability of FLASH systems that include minimally modified clinical settings, well-defined beam characteristics and FLASH-enabled treatment planning system (TPS). In this study, we present a potential solution with rigorous commissioning of a modified common LINAC and implementation of an electron FLASH beam model in a widely-adopted TPS.
Methods: A Varian Clinac 2100C/D was converted to deliver UHDR electron beams by withdrawing the target and scattering foil while running in 10MV x-ray mode. Beam characteristics and stability were quantified by film, Cherenkov and scintillation imaging. The eFLASH beam model, established in Geant4 and implemented in Varian Eclipse TPS, was validated by comparing calculations with film measurements. eFLASH plans were generated for representative canine and human patient cases to demonstrate the planning and delivery capability in minimally modified clinical settings involving complex geometries and anatomical inhomogeneities.
Results: The surface average dose rate at the isocenter was >230Gy/s for all measured fields with adequate long-term stability (deviations of output <10%, symmetry/flatness <2%, spatial shift and shape <2mm). The Geant4 beam model and its Eclipse implementation were validated to clinical accuracy (absolute average error was <1.5% for lateral profiles, and <2% for percent-depth-dose profiles). An eFLASH-RT plan was generated and accurately delivered to a canine patient with a malignant melanoma tumor in the posterior oral cavity. By utilizing routine accessories, oblique gantry angle and couch kick, a human eFLASH-RT plan comparable to a conventional electron plan was achieved.
Conclusion: Treatment planning and accurate delivery of eFLASH-RT were demonstrated feasible in minimally modified radiation oncology clinical settings. The presented solution and its open-source beam model are readily transferable to facilitate the clinical translation of eFLASH-RT.