Room: Track 3
Purpose: Ultra-high dose rate in radiotherapy (FLASH) has been shown to increase the therapeutic index, but its clinical translation is hindered by the challenge to intensity-modulate the high-dose-rate X-rays. In this study, we develop the ROtational direct Aperture optimization with a Decoupled ring-collimator (ROAD) to achieve simultaneous ultrafast delivery and complex dose modulation and evaluate the biological gain with a radiolytic oxygen depletion model.
Methods: The ROAD design includes a fast-rotating slip-ring linac and a decoupled ring-collimator with 75 pre-shaped multi-leaf-collimator (MLC) modules. The source-ring and collimator-ring rotate at 1 rps clockwise and counterclockwise, respectively. The X-ray source is triggered when the source is aligned with individual MLCs, achieving 150 equal-angular beams for one full arc. The Direct Aperture Optimization (DAO) for ROAD was formulated to include a least-squares dose fidelity objective, an anisotropic total variation term, and a single segment term encouraging simple apertures. The biological equivalent dose (BED) was computed voxelwise using a spatiotemporal model accounting for cell respiration, tissue diffusion, and radiolytic oxygen depletion. ROAD was compared with clinical Volumetric Modulated Arc Therapy (VMAT) on both physical dose and BED of a brain, a lung, a prostate, and a head and neck cancer patient.
Results: The ROAD plans can be delivered within 1s, achieving 112 Gy/s average per-beam dose rate. ROAD improved the PTV homogeneity by 3%, and reduced OAR max and mean physical dose by 4.8Gy and 6.3Gy compared with clinical VMAT. The OAR max and mean BED were reduced by 13Gy and 6.6Gy. The average R50 and integral dose of [VMAT, ROAD, ROAD-BED] are [4.8, 3.3, 1.7] and [88, 56, 43] Gy×Liter respectively.
Conclusion: The novel ROAD method improves physical dosimetry compared with clinical VMAT and provides huge potential in biological gains from the FLASH effect.
Funding Support, Disclosures, and Conflict of Interest: This work is supported by NIH Grants Nos. R01CA188300, R43CA183390, and R44CA183390, and DOE Grants Nos. DE-SC0017057 and DE-SC0017687.