Field size consistency of nominally matched linacs

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Dosimetry

Background:

• Sub-millimetre changes in collimator position significantly affect radiotherapy beam dosimetry.
• Understanding variability in small field collimation between linear accelerators (linacs) is critical for accurate treatment planning.

Purpose of the Study:

• To evaluate potential variability in small field collimation between dosimetrically matched Varian iX linacs.
• To assess the impact of collimator type (jaws vs. multi-leaf collimator) on field size and output factor consistency.

Main Methods:

• Measured field sizes and output factors for jaw- and MLC-collimated fields using radiochromic film and an electron diode.
• Utilized nominal field sizes ranging from 0.6x0.6 cm² to 10x10 cm² across eight Varian iX linacs.
• Measurements were performed at a zero gantry angle (beam directed vertically downward).

Main Results:

• Measured field size differences between linacs exceeded measurement and mechanical uncertainties for some fields.
• MLC-defined fields showed greater inter-linac consistency but larger deviations from nominal sizes compared to jaw-defined fields.
• Field output factors became consistent between linacs for nominal field sizes of 2x2 cm² and larger.

Conclusions:

• Caution is advised when using jaw-collimated small field output factors in treatment planning systems due to reproducibility issues.
• Thorough investigation of small field size and reproducibility is essential during linac and treatment planning system commissioning.
• Further studies with different linac models, collimation systems, and beam orientations are recommended.