Modeling the Agility MLC in the Monaco treatment planning system

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Dosimetry

Background:

• Accurate dose calculation in radiotherapy relies heavily on precise modeling of the linear accelerator's Multi-Leaf Collimator (MLC).
• The Monaco treatment planning system (TPS) utilizes a sophisticated MLC model that requires careful parameterization for specific linac hardware, such as the Elekta Agility MLC.

Purpose of the Study:

• To investigate the relationship between various parameters in the Monaco MLC model and dose calculation accuracy for an Elekta Agility MLC.
• To determine the impact of different MLC modeling parameter sets on dose distributions and point-dose measurements using both vendor-provided and in-house procedures.

Main Methods:

• MLC modeling was performed using vendor-provided and in-house procedures for an Elekta Agility MLC within the Monaco TPS.
• Simple and complex treatment plans (IMRT, VMAT) based on TG-119 structure sets were used for evaluation.
• Dosimetric measurements were acquired using EDR2 film, MapCHECK, and ion chambers.

Main Results:

• The vendor-determined MLC parameter set provided acceptable gamma pass rates but lacked point-dose accuracy compared to other systems.
• Multiple parameter sets, including those with opposing adjustments to leaf offset and tip leakage, yielded similar dosimetric characteristics.
• Gamma pass rates were less sensitive to parameter choices than point-dose measurements, suggesting gamma evaluation alone may be insufficient for MLC model validation.

Conclusions:

• MLC modeling parameter optimization is critical for achieving accurate dose calculations in the Monaco TPS with the Elekta Agility MLC.
• A combination of vendor-provided and in-house procedures leads to the most accurate models.
• Over-modeling to match specific quality assurance fields can compromise accuracy for typical clinical dose distributions.