Small field dosimetry on a ring-shape Linac machine with a dual-layer MLC.

Area of Science:

• Medical Physics
• Radiation Oncology

Background:

• The Varian Halcyon linear accelerator (linac) with a dual-layer MLC and 6 MV FFF beam is increasingly used for treating small target volumes.
• Accurate dose calculation for small static fields on this platform is a significant concern.

Purpose of the Study:

• Investigate the dosimetric accuracy of small static fields from the Halcyon dual-layer MLC.
• Evaluate the modeling of these fields in Eclipse using Acuros XB (AXB) versions 16.1, 18.0, and 18.1.

Main Methods:

• Measured on-axis field output factors (FOFs) for small fields (0.5-4 cm) using microDiamond and microSilicon detectors per TRS-483.
• Compared detector measurements with AXB calculations in Eclipse v16.1, v18.0, and v18.1.
• Analyzed profiles, penumbrae, and evaluated a clinical brain VMAT plan.

Main Results:

• Detector agreement was within ±0.5%; differences from vendor data were 1.1%-1.8% for 1 cm fields.
• AXB v16.1 underestimated FOFs by up to -27% for 0.5x0.5 cm² fields; v18.x agreed within ~2%.
• Y-penumbra modeling was incomplete, and v16.1 showed central underdosage in a clinical case with small fields.

Conclusions:

• Acuros XB versions 18.x and later significantly improve small-field accuracy for the Halcyon linac.
• Incomplete Y-penumbra modeling persists, highlighting the need for careful treatment planning and optimization constraints for small fields.