Geometric and dosimetric analysis of multileaf collimation conformity

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Physics

Background:

• Concerns exist regarding multileaf collimator (MLC) field edge conformity compared to cerrobend blocks.
• Users remain cautious with MLCs for small fields despite dispelling reports.
• Leaf orientation poses challenges for conformity in dynamic or universal wedge applications.

Purpose of the Study:

• To investigate the impact of field size and elongation on MLC field conformity.
• To compare geometric and dosimetric conformity of MLCs for various field shapes.

Main Methods:

• Geometric analysis of overblocking and underblocking areas for circles and ellipses.
• Dosimetric evaluation using radiographic films at depth in the beam's eye view plane.
• Examined leaf orientation along major and minor axes of ellipses with varying eccentricity.

Main Results:

• Geometric non-conformity increases rapidly as circle diameter decreases.
• For ellipses, conformity improves with increasing dimension of the axis perpendicular to leaf motion.
• Dosimetric analysis revealed decreased predictability due to scatter, leaf inaccuracies, and uncertainties.

Conclusions:

• Exercise caution when using MLCs for small round fields.
• Align MLC leaf movement with the minor axis for elliptical fields to maximize conformity.
• Thinner MLC leaf width may enhance conformity for small round fields.