Treatment Planning System for Electron FLASH Radiation Therapy: Open-Source for Clinical Implementation.

Area of Science:

• Medical Physics
• Radiation Oncology
• Computational Modeling

Background:

• Ultrahigh dose-rate electron FLASH radiation therapy (eFLASH-RT) offers potential clinical advantages.
• Accurate beam modeling in treatment planning systems (TPS) is crucial for clinical implementation.
• Existing TPS may not fully support novel eFLASH-RT techniques.

Purpose of the Study:

• To develop and implement a Monte Carlo (MC) beam model for a modified ultrahigh dose-rate electron FLASH linear accelerator (LINAC) in a clinical TPS (Varian Eclipse).
• To validate the MC beam model using clinical measurements and assess its accuracy for eFLASH-RT.
• To evaluate the clinical planning capabilities of the implemented model for both preclinical and human patient cases.

Main Methods:

• Modeled the LINAC head using the Geant4-based GAMOS MC toolkit.
• Varied beam parameters (energy spectrum, emittance cone angle) to match measured percent depth dose (PDD) and lateral profiles.
• Validated the model and its Eclipse configuration with clinical applicator measurements and performed MC forward dose calculations.
• Compared eFLASH-RT and conventional (Conv-) RT electron plans for a human patient.

Main Results:

• The eFLASH beam model achieved best agreement with measurements at specific energy spectrum and emittance cone angle parameters.
• The model and TPS configuration were validated to clinically acceptable accuracy, with small average errors in PDD and lateral profiles.
• MC calculations demonstrated adequate dose delivery for a mouse whole-brain treatment and comparable plan quality for a human patient case.
• The eFLASH-RT plan achieved 90% tumor coverage with 95% of the prescribed dose, comparable to Conv-RT.

Conclusions:

• This study presents the first functional MC beam model for eFLASH-RT commissioned in a clinical TPS, enabling planning and evaluation.
• The developed model minimizes deviation from conventional RT workflows, facilitating clinical translation.
• The comparable plan quality between eFLASH-RT and Conv-RT in a complex human patient case highlights the model's clinical utility.
• The methodology can be extended to model other eFLASH irradiators with varying beam characteristics.