Author: Quyen Duong, Si Young Jang, Soyoung Lee, Samuel Ming Ho Luk 👨🔬
Affiliation: Boston University, Department of Radiation Oncology, University of Vermont Medical Center, Boston Medical Center 🌍
Purpose: To investigate the clinical impact of physical MLC gaps changes with Millennium 120 MLC on Enhanced Leaf Modeling (ELM) and patient plans, and to establish a new MLC model in Eclipse v18.0 on two beam-matched TrueBeam treatment units.
Methods: In order to implement the ELM with equivalent dosimetry on the beam-matched TrueBeam units, an additional physical MLC gap adjustment was applied to Millennium120 MLC. The impact of 0.3 mm of the physical gap adjustment on the dosimetric leaf gap was measured. MLC parameters such as Leaf Gap were acquired for ELM in Eclipse v18.0. Dosimetric impacts on sweeping gap MLC fields and patient’s plans with IMRT, VMAT, SBRT and SRS-HyperArc were investigated.
Results: The calculated dosimetric leaf gap was 0.163 (6MV, 0.148 for 6FFF) and 0.132 cm (6MV, 0.117 for 6FFF) and the derived leaf gap was -0.020 and -0.051 cm for 6MV (AAA&Acuros), -0.026 and -0.057 cm for 6FFF (AAA&Acuros) with the physical leaf gap set at 0.05 and 0.02 cm, respectively. A physical MLC gap adjustment from 0.5 to 0.2 mm resulted in -5.1, -5.1, -5.3, and -5.7% dose difference at the center of 2 cm MLC field with 3 mm sweeping gaps at the central beam axis, 4, 8, and 12 cm off-axis using an ionization chamber (0.053 cm3), respectively. All patient specific QAs (total 51-field) with SRSMapCheck or portal met Gamma criteria >95% (3%/1mm for SBRT/SRT) and >97% (3%/3mm for IMRT/VMAT). The maximum dose differences in the patient’s plans were up to -5.6% in static IMRT using dynamic MLC, -0.5% in VMAT, 1.4% in SBRT, 2.3% in HyperArc.
Conclusion: An enhanced MLC model in Eclipse v18.0 showed excellent agreement between measurements and calculations. An accurate MLC gap in the ELM is critical in patient’s plans, especially for IMRT cases using dynamic dose delivery by MLCs.