Clinically shaped fields: relative outputs for Varian multileaf collimators and tray-mounted cerrobend blocks

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Physics

Background:

• Multileaf collimators (MLCs) and cerrobend blocks are used to shape radiation beams in radiotherapy.
• Variations in relative output factors between these two methods can impact dose delivery accuracy.

Purpose of the Study:

• To quantify the variation in relative output factors between MLC-shaped fields and cerrobend block-shaped fields.
• To compare the dosimetric consistency of MLCs versus traditional blocks in clinical scenarios.

Main Methods:

• Compared 21 clinical fields from 16 patients using Varian linear accelerators (2300CD and 2100CD).
• Measured output factors in solid water at specified depths and source-to-axis distance (SAD = 100 cm) for 6 and 20 MV photon beams.
• Utilized cerrobend blocks cast from films that generated the MLC shapes for direct comparison.

Main Results:

• The overall average MLC factor (MLC reading/block reading) was 0.9952 with a standard deviation of 0.0024.
• Clinical average MLC factors were 0.9957 (SD=0.0022), consistent across different energies and depths.
• Repeat measurements on a different MLC model showed results within 0.2% of initial findings.

Conclusions:

• MLC-shaped fields demonstrate high consistency and minimal variation compared to cerrobend blocks.
• MLCs provide a reliable and accurate method for field shaping in radiotherapy, comparable to traditional blocks.
• Further research may explore correlations between MLC factors and open field percentages.