Physical characterization and comparison of two commercially available micro-MLCs

Abstract 

In this study, the physical characteristics (penumbra width variation with the source size and shape, interleaf leakage, transmission through the leaves, and the tongue-and-groove effect) of two linear accelerators (BrainLAB's Novalis and Elekta's Synergy-S Beam Modulator) have been investigated. For similar square fields (about 4.5cm×4.5cm) with source-to-surface/skin-distance (SSD) ranging from 90cm to 115cm and measurements taken at the depth of D_max=1.5cm for 6MV photon beam. The Novalis MLC has penumbra width of 2.4±0.11mm–2.8±0.11mm at the leaf-end and 2.2±0.1mm–2.7±0.1mm on the leaf-side; and those for the Synergy-S MLC are 4.4±0.17mm–5.2±0.2mm and 3.0±0.12mm–3.5±0.12mm. Upon rotating the Synergy-S collimator by 90° (i.e., shifting the leaf movement to the gun–target direction), significant reduction of the leaf-end penumbra width (17%) and increase of leaf-side penumbra width (28%) suggest an elliptical shape of the radiation source spot. Similar rotation of the collimator yielded reduction of the penumbras on both leaf-end (34%) and leaf-side (28%) for Novalis, indicating that the Novalis has a more symmetric source size. For all the field sizes and settings, BrainLAB's Novalis μMLC produce a smaller penumbra for simple square fields compared to the Elekta's Synergy-S. However, this difference became less pronounced for leaf-side penumbra and also for circular fields. The tongue-and-groove effect of the Novalis (23±0.9%) is slightly smaller than that of the Synergy-S (25±1%); while the interleaf leakage and leakage directly through leaves for Synergy-S (1.6±0.07% & 0.9±0.04%) are lower than that of Novalis (2±0.08% & 1.3±0.05%).

Keywords: Micromulti-leaf collimator, Penumbra, Interleaf leakage, Tongue-and-groove effect, Focal spot, IMRT