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Proton Therapy Delivery and Its Clinical Application in Select Solid Tumor Malignancies

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

Y Yang¹, J Wong¹, T McNutt¹

¹Johns Hopkins University, Baltimore, MD.

Medical Physics

|May 19, 2017

Summary

Utilizing 2mm gaps in the unused leaf-pairs during intensity-modulated radiation therapy (IMRT) delivery significantly reduces treatment time. This method accelerates beam delivery with acceptable increases in target dose and minimal impact on critical structures.

Keywords:

Accelerated beams Collimators Field shaping Intensity modulated radiation therapy Medical treatment planning Modulators Multileaf collimators

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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.