Optimal VMAT Delivery for Elekta MLC Beam Modulator: A Study of Collimator Rotation for Head and Neck Planning

Background Well-optimized treatment planning parameters are vital for optimum beam delivery in advanced radiotherapy techniques. The Elekta “Beam-Modulator” (BM) is a high-resolution multileaf collimation system where each individual leaf is 4 mm wide at the isocentre, without backup diaphragms and jaws. Its maximum aperture is 21 × 16 cm2, which results in a limited clinical use for the target geometry of maximum 20 cm in length. The collimator rotation provides an opportunity to treat slightly extended treatment length with optimal target coverage. The study aims to observe the collimator rotation influence on volumetric modulated arc therapy (VMAT) plan quality for different head and neck target geometries using limited field collimator of BM. Methods Ten patients with head and neck cancer were planned by means of simultaneous integrated boost to deliver VMAT for five patients with three dose levels (70/60/56) and five patients with two dose levels (60/54). The single arc, dual arc, and combined two independent single arcs of 356° each were well optimized for four collimator angles (C) 15°,30°, 45°, and 90°. The plans were prepared for BM with SmartArc module of Pinnacle³ treatment planning system. Statistical significance (P ≤ .05) among collimator angles for planning target volume dose-volume indices was calculated with Student's t-test. Organ-at-risk doses were compared and monitor units were also evaluated as a parameter for dose-delivery efficiency and out-of-field dose index. Results The dual arc and combined two independent single arcs achieved planning objectives for C15°, C30°, and C45°. Single arc for all collimator angles and C90° for all VMAT schemes failed to achieve planning objectives. The spread of low dose bath 20, 35, and 40 Gy and deterioration of doses were higher towards periphery at C90° and statistically significant. Conclusion The small and medium collimator angles for dual-arc VMAT scheme(s) are suitable, whereas single arc and C90° are not suitable in VMAT implementation for Elekta Beam-Modulator collimation system.