Physical characterization and comparison of two commercially available micro-MLCs

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 4.5 cm × 4.5 cm) with source-to-surface/skin-distance (SSD) ranging from 90 cm to 115 cm and measurements taken at the depth of Dmax = 1.5 cm for 6 MV photon beam. The Novalis MLC has penumbra width of 2.4 ± 0.11 mm–2.8 ± 0.11 mm at the leaf-end and 2.2 ± 0.1 mm–2.7 ± 0.1 mm on the leaf-side; and those for the Synergy-S MLC are 4.4 ± 0.17 mm–5.2 ± 0.2 mm and 3.0 ± 0.12 mm–3.5 ± 0.12 mm. 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%).