Investigation on the impact of the leaf trailing effect using the Halcyon integrated platform system

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Treatment Planning

Background:

• The double-stacked Halcyon multileaf collimator (MLC) introduces complexities in treatment planning systems (TPSs).
• The leaf trailing effect, arising from interactions between MLC layers, impacts dosimetric leaf gap (DLG) and is not accurately modeled by the Eclipse TPS.

Purpose of the Study:

• To investigate and quantify dose discrepancies in clinical plans due to Halcyon MLC limitations in the Eclipse TPS.
• To explore methods for mitigating and avoiding these dose discrepancies in radiotherapy treatments.

Main Methods:

• Utilized an integrated platform (Halcyon v2, Eclipse, Aria v15.6) for analysis.
• Dose discrepancies were measured using electronic portal imaging device (EPID) and validated with radiochromic film.
• Analyzed clinical beams using sliding window dynamic IMRT (dIMRT) and modified plans by adjusting leaf retraction to alter trailing distance.

Main Results:

• Significant dose discrepancies (>10%) and gamma failures were observed in both commissioning tests and clinical dIMRT plans.
• Discrepancies were attributed to limitations in the TPS's MLC model, particularly with slow-moving or static leaves.
• Increasing trailing distance by retracting noncollimating leaves partially mitigated discrepancies but was not universally applicable.

Conclusions:

• Eclipse TPS limitations with the Halcyon MLC model lead to dose calculation errors, causing QA failures and workflow disruptions.
• Clinical plan evaluation is crucial, especially for static-gantry dIMRT, and problematic leaf sequence characteristics should be avoided.
• Improved MLC modeling is essential for accurate and reliable dose calculations, particularly in adaptive radiotherapy.