A light field-based method to adjust rounded leaf end MLC position for split shape dose calculation correction in a...

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Treatment Planning

Background:

• Intensity-modulated radiation therapy (IMRT) relies on precise dose calculations for effective cancer treatment.
• Multileaf collimators (MLCs) shape radiation beams, but their rounded leaf ends can introduce dose calculation inaccuracies, particularly in split-field scenarios.
• Existing treatment planning systems may not fully account for these geometric complexities, potentially leading to significant dose errors.

Purpose of the Study:

• To investigate and analytically/experimentally study split-field dose calculation corrections in IMRT.
• To develop a method for adjusting multileaf collimator (MLC) leaf positions to enhance dose calculation accuracy.
• To quantify the potential under- or overdosage errors without correction and the achievable accuracy with the proposed correction method.

Main Methods:

• Derived the precise light field edge position (Xtang.p) using geometric and mathematical methods based on 50% central axis dose.
• Related Xtang.p to the MLC leaf position (Xmlc.p) used in treatment planning systems for monitor unit calculation.
• Quantified the offset correction by calculating the difference between the 50% dose position and Xmlc.p across various source-surface distances (SSD) and energies (6 MV, 10 MV).

Main Results:

• The 50% dose position was found to be outside Xmlc.p within specific MLC leaf position ranges (+8 cm to -8 cm), leading to positive offset corrections.
• Conversely, negative offset corrections were observed when the 50% position was inside Xmlc.p in other ranges (-12 cm to -8 cm and +20 cm to +8 cm).
• Without offset correction, monitor unit calculations could result in dose errors of up to 7.5% per mm. Calibration at a specific SSD could generalize offset corrections for all SSDs.

Conclusions:

• Accurate offset correction, derived from precise geometric analysis and experimental validation, is crucial for split-field dose calculations in IMRT.
• The proposed method allows for accurate determination of MLC leaf edge locations, significantly reducing dose calculation errors.
• With careful implementation, this approach can achieve dose calculation accuracy within 0.5% for adjusted MLC leaf edges in IMRT treatment planning.