SU-E-T-385: Accelerated Beam Delivery with MLC Gaps in IMRT Fields.

Area of Science:

• Medical Physics
• Radiation Oncology

Background:

• The Elekta Beam Modulator Multi-Leaf Collimator (MLC) typically retracts unused leaf-pairs to minimize leakage dose.
• This retraction increases beam delivery time, particularly for intensity-modulated radiation therapy (IMRT).

Purpose of the Study:

• To investigate the dosimetric impact of allowing unused MLC leaf-pairs to remain within the planning target volume (PTV) aperture as a 2mm gap.
• To evaluate the potential for improved delivery efficiency in IMRT by utilizing these gaps.

Main Methods:

• The Pinnacle treatment planning system's capability to model dose through 2mm MLC gaps was assessed.
• EBT film measurements in a solid water phantom were compared to computed doses.
• Dosimetric features and delivery times of IMRT plans with and without MLC gaps were compared for multiple cases.

Main Results:

• Computed dose through MLC gaps was dependent on dose grid resolution; a 0.5mm grid yielded <1% difference for central axis and <3% for off-axis gaps compared to measurements.
• The maximum dose through a 2mm x 4mm leaf gap was <10cGy for a typical IMRT plan.
• IMRT plans with gaps showed up to a 2% increase in target dose with minimal changes to critical structures.
• Beam delivery time was reduced by an average of 27% across 13 IMRT cases.

Conclusions:

• Utilizing minimal 2mm gaps in IMRT fields accelerates beam delivery.
• The resulting increase in target dose is clinically acceptable, with negligible effects on surrounding critical structures.