Technical Note: Plan-delivery-time constrained inverse optimization method with minimum-MU-per-energy-layer (MMPEL)...

Area of Science:

• Medical Physics
• Radiation Oncology
• Computational Optimization

Background:

• Pencil beam scanning (PBS) proton therapy is a precise cancer treatment modality.
• Treatment delivery time is a significant factor in PBS, largely determined by dose delivery time.
• Current methods for optimizing PBS delivery time are limited by a single, global minimum monitor unit (MU) threshold, restricting potential time savings and potentially compromising plan quality.

Purpose of the Study:

• To develop and validate a novel method to reduce the dose delivery time of PBS proton plans.
• To address the limitations of global MU thresholds by introducing variable, energy-layer-specific thresholds.
• To jointly optimize treatment plan delivery time and plan quality in PBS proton therapy.

Main Methods:

• Introduction of minimum-MU-per-energy-layer (MMPEL), a novel approach using variable MU thresholds adapted to each energy layer.
• Formulation of MMPEL as a constrained optimization problem, balancing dose-volume-histogram (DVH) based planning constraints, plan delivery time, and minimum MU constraints per energy layer.
• Solution of the MMPEL problem using iterative convex relaxations via the alternating direction method of multipliers (ADMM).

Main Results:

• MMPEL demonstrated substantial reductions in dose delivery time across multiple cancer sites (prostate, lung, brain, head-and-neck, breast, liver, pancreas), ranging from 14% to 67%.
• Accepting minor, physician-approved degradations in plan quality further reduced treatment times by an additional 11% to 41%.
• Overall, MMPEL accelerated PBS plan delivery by 2-10 fold compared to conventional methods.

Conclusions:

• The developed MMPEL method offers a significant advancement in PBS proton therapy planning.
• Variable, energy-adaptive MU thresholds effectively optimize dose delivery time while maintaining acceptable plan quality.
• MMPEL shows considerable potential for substantially reducing treatment times in clinical practice.