SU-E-T-21: Modeling a MLC Scatter Source for In-Air Output Factors

Area of Science:

• Medical Physics
• Radiation Oncology

Background:

• Scattered radiation from multi-leaf collimators (MLCs) impacts in-air output ratio (Sc) calculations for small and irregular fields used in intensity-modulated radiation therapy (IMRT).
• Accurate Sc calculation is crucial for precise radiation dose delivery in IMRT.

Purpose of the Study:

• To develop and validate an extra-focal source model for MLC-scattered radiation to enhance Sc calculation accuracy.
• To improve the precision of radiation dosimetry for IMRT applications.

Main Methods:

• A conventional dual-source model was established using measured Sc data for collimator-defined fields.
• An MLC scatter source, modeled by two Gaussian functions, was integrated and optimized against measured Sc data for various MLC fields.
• The developed model's accuracy was evaluated by comparing calculated Sc values with measurements for diverse IMRT fields.

Main Results:

• The developed MLC scatter source model demonstrated a mean discrepancy of 0.08±0.28% between calculated and measured Sc.
• The conventional dual-source model showed a higher mean discrepancy of 0.44±0.39%.

Conclusions:

• The proposed MLC scatter source model, combined with a dual-source model, significantly enhances the accuracy of Sc calculations for IMRT fields.
• This advancement contributes to more reliable dose calculations in complex radiation therapy techniques.