Robust optimization in IMPT using quadratic objective functions to account for the minimum MU constraint.

Area of Science:

• Medical Physics
• Radiation Oncology
• Computational Optimization

Background:

• Current clinical practice for intensity-modulated proton therapy (IMPT) applies minimum monitor unit (MU) constraints post-optimization, potentially compromising plan quality and robustness.
• This conventional approach may lead to suboptimal treatment plans due to the delayed consideration of MU constraints.

Purpose of the Study:

• To develop and evaluate a novel method that directly incorporates the minimum MU constraint into the robust optimization process for IMPT.
• To mitigate the negative impacts of minimum MU constraints on plan quality and robustness in IMPT.

Main Methods:

• A new quadratic optimization approach was developed to integrate the minimum MU constraint directly into robust optimization.
• The method simultaneously considered the impact of uncertainties and the minimum MU constraint.
• The approach was validated using treatment data from seven cancer patients with varying machine settings.

Main Results:

• The novel method demonstrated superior plan quality compared to the conventional approach, evidenced by improved D95% of the clinical target volume (CTV).
• CTV D95% coverage was 99.4% (99.2%-99.6%) with the new method versus 99.2% (98.6%-99.6%) with the conventional method.
• Plan robustness, assessed by the CTV dose-volume histogram band gap at D95%, was comparable between the two methods (1.5% [0.5%-4.3%] vs. 1.2% [0.6%-3.8%]).

Conclusions:

• Directly integrating the minimum MU constraint into robust optimization for IMPT yields machine-deliverable plans.
• The new method enhances tumor coverage while preserving a high degree of plan robustness.
• This optimized approach offers a significant advancement in IMPT treatment planning.