Simultaneous dose and dose rate optimization (SDDRO) for FLASH proton therapy.

Area of Science:

• Radiation Oncology
• Medical Physics
• Computational Biology

Background:

• FLASH radiotherapy (RT) aims to reduce normal tissue toxicity by using ultra-high dose rates (≥40 Gy/s).
• Current proton RT treatment planning optimizes dose distribution but not dose rate, limiting FLASH's potential.
• Proton RT is a viable modality for achieving the high dose rates required for FLASH techniques.

Purpose of the Study:

• To develop a novel treatment optimization method for FLASH proton RT that optimizes both dose and dose rate distributions simultaneously.
• To introduce Simultaneous Dose and Dose Rate Optimization (SDDRO) to enhance the therapeutic ratio in FLASH proton therapy.
• To address the limitation of existing methods that only optimize dose distribution, not dose rate.

Main Methods:

• SDDRO is formulated as a constrained optimization problem incorporating dose-volume constraints and minimum dose rate constraints.
• The optimization algorithm employs iterative convex relaxation and the alternating direction method of multipliers.
• Algorithms were developed for both constant and variable beam intensity scenarios.

Main Results:

• SDDRO significantly improved dose rate distribution compared to intensity modulated proton therapy (IMPT).
• The volume of interest receiving ≥40 Gy/s increased from 30-50% to ≥98% with SDDRO.
• Lung volume receiving ≥40 Gy/s increased from 30-40% to 70-90% with SDDRO.
• Hypofractionation and multiple beams further enhanced SDDRO plan quality for both dose and dose rate distributions.

Conclusions:

• A novel joint dose and dose rate optimization method, SDDRO, was developed for FLASH proton RT.
• SDDRO substantially improves FLASH dose rate coverage, enhancing normal tissue sparing while preserving dose distribution.
• Combining hypofractionation and multiple beams can further optimize SDDRO plan quality.