Characterization of the Halcyon^TM multileaf collimator system

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Technology

Background:

• The Halcyon system features a novel stacked and staggered dual-layer multileaf collimator (MLC).
• Understanding its performance is crucial for optimizing radiation therapy delivery.
• Comparison with the established Millennium 120-leaf MLC provides valuable context.

Purpose of the Study:

• To characterize the performance of the stacked and staggered dual-layer MLC on the Halcyon system.
• To evaluate its positioning stability, leaf transmission, penumbra, and edge/end effects.
• To assess dose prediction accuracy using different calculation models.

Main Methods:

• Machine Performance Check (MPC) data analyzed for MLC positioning stability over 70 days.
• Leaf transmission measured using a Farmer chamber comparing open and closed fields.
• Penumbra, leaf end effect, and leaf edge effect evaluated using film, sweeping gap fields, and Electronic Portal Imaging Device (EPID).
• Point dose measurements compared against treatment planning system (TPS) predictions.

Main Results:

• MLC positioning accuracy deviation within 0.1 mm; proximal MLC showed greater deviations than distal.
• Dual-layer MLC significantly reduced inter-leaf and intra-leaf transmission compared to single-layer.
• Leaf transmission was negligible for the dual-layer combination.
• Leaf end penumbra was wider than leaf edge penumbra; leaf end effect measured at -0.2 mm.
• Leaf edge effect showed minimal angular dependence, but systematic deviations from TPS were noted.
• Dose prediction accuracy improved with the latest dose calculation algorithm version.

Conclusions:

• The stacked and staggered dual-layer MLC on the Halcyon system exhibits favorable characteristics for clinical implementation.
• The system demonstrates high precision in leaf positioning and reduced radiation leakage.
• Further investigation into TPS modeling of leaf edge effects is warranted.