ROAD: ROtational direct Aperture optimization with a Decoupled ring-collimator for FLASH radiotherapy

Area of Science:

• Medical Physics
• Radiotherapy Engineering
• Radiation Oncology

Background:

• Ultra-high dose rate (FLASH) radiotherapy demonstrates a higher therapeutic index by reducing normal tissue toxicity while maintaining tumor cell killing.
• Achieving effective X-ray FLASH requires intensity modulation to ensure dose conformity, preventing biological gains from being offset by physical dose limitations.
• Current methods face challenges in delivering the necessary high dose rates with precise dose modulation for X-ray FLASH.

Purpose of the Study:

• To develop and evaluate a novel system, the ROtational direct Aperture optimization with a Decoupled ring-collimator (ROAD), for simultaneous ultrafast dose delivery and complex dose modulation in X-ray FLASH radiotherapy.
• To compare the dosimetric performance of the ROAD system against conventional clinical volumetric modulated arc therapy (VMAT).

Main Methods:

• The ROAD system integrates a fast-rotating slip-ring linear accelerator (linac) with a decoupled collimator ring featuring 75 multi-leaf-collimator (MLC) modules.
• Direct Aperture Optimization (DAO) was formulated for ROAD, incorporating dose fidelity, anisotropic total variation, and single segment terms.
• FLASH dose (FD) and FLASH biological equivalent dose (FBED) were calculated voxelwise, with FBED utilizing a spatiotemporal model for radiolytic oxygen depletion.

Main Results:

• The ROAD system achieved mean dose rates of 76.2 Gy/s (ROAD-75) and 112 Gy/s (ROAD-150) for delivering a 25 Gy single-fraction dose in 1 second.
• Compared to VMAT, ROAD-150 demonstrated improved planning target volume (PTV) homogeneity and reduced organ at risk (OAR) physical dose (max: 4.8 Gy, mean: 6.3 Gy).
• ROAD systems significantly reduced average R50 and integral dose (ROAD-75: 3.2 Gy, 57 Gy*L; ROAD-150: 3.2 Gy, 56 Gy*L) compared to VMAT (4.8 Gy, 89 Gy*L).

Conclusions:

• The novel ROAD design successfully achieves ultrafast dose delivery and enhances physical dosimetry compared to clinical VMAT.
• The system offers a potentially viable engineering solution for implementing X-ray FLASH radiotherapy.
• Calculated FD and FBED indicate model-dependent FLASH effects, highlighting the importance of advanced modeling in treatment planning.