Multileaf collimation versus alloy blocks: analysis of geometric accuracy

Area of Science:

• Medical Physics
• Radiation Oncology

Background:

• Custom lead alloy blocks have been traditionally used for radiation shielding in cancer treatment.
• The finite resolution of multileaf collimators (MLCs) introduces geometric displacement errors.
• Accurate beam shaping is critical for effective radiation therapy and minimizing off-target dose.

Purpose of the Study:

• To compare the fabrication-related misalignment errors of custom lead alloy blocks with the displacement errors from multileaf collimator (MLC) resolution.
• To evaluate the geometric accuracy of MLCs against custom blocks in relation to physician-prescribed radiation therapy apertures.

Main Methods:

• Apertures from custom block fabrication stages and corresponding MLC apertures were collected for randomly selected patients across four clinical sites.
• Deviations from physician-prescribed smooth apertures were measured for both custom blocks and MLCs.
• Errors from block fabrication were compared with geometric displacements from MLC leaf width.

Main Results:

• Multileaf collimator (MLC) conformity is dependent on the treatment site and field shape.
• For three of four sites, MLC apertures tracked prescribed apertures as accurately as, or better than, custom blocks.
• This comparison considered the entire custom block process, including design, fabrication, mounting, and alignment.

Conclusions:

• The geometric conformality of multileaf collimation is comparable to custom blocks.
• In certain clinical scenarios, multileaf collimation demonstrates superior geometric accuracy compared to custom blocks.