Author: Evan Barber, Gloria P. Beyer, Callum Hartley, Jordi Saez, Philip Wheeler 👨🔬
Affiliation: Department of Radiation Oncology, Hospital Clinic de Barcelona, Department of Radiotherapy Physics, Velindre University NHS Trust, Medical Physics Services, LLC 🌍
Purpose: To evaluate the Enhanced Leaf Model (ELM) in Eclipse v18 to the standard leaf model for the Halcyon multileaf collimator (MLC) using a measurement-based method.
Methods: The Halcyon’s double-layer MLC, with rounded leaf ends, presents a unique challenge for modeling due to the "trailing effect," where the distance between the leaf tips of both layers modifies the composite leaf-end shape. Recently, Varian introduced the ELM, which incorporates attenuation maps calculated via ray tracing for improved modeling of the leaf tip. This study assessed its impact on trailing effect modeling.
Python-generated sweeping gap plans simulated motions for one MLC layer, with the trailing layer maintaining offsets of 0, 2, and 20 mm. Gaps of 2, 5, 10, 15, and 20 mm were evaluated for both lower and upper layers leading. Isocentric measurements were performed using a Farmer chamber. The mean dose to the ion chamber volume was calculated using AAA and AcurosXB in Eclipse v15.6 and v18. Using portal dosimetry, two fluence patterns and two clinical dynamic fields were also analyzed.
Results: Calculated doses with the previous MLC model varied minimally with trailing distance. This led to significant differences from measured values of up to 40% for a 2 mm gap and above 2% for most gap sizes and trailing distances, even as the trailing distance increased. In contrast, the ELM calculations resulted in dose variation with trailing distance, improving agreement with measured values. Differences with predicted values were below 2% for all configurations except the 2mm gap. Clinical plan analysis with Portal Dosimetry confirmed improved dose accuracy in regions with minimal distance between MLC tips.
Conclusion: The new ELM in Eclipse shows improved modeling of the leaf tip correctly, taking into account the interaction between the leaf tips of both layers in trailing sweeping gap tests and clinical scenarios.