Penumbra evaluation of the synergy-S and novalis Micro-MLCs

Purpose: To investigate the effect of variations in the field size, treatment depth, and treatment distance on penumbra at leaf‐end and leaf‐side of two micro‐leaf collimators (μMLCs). Method and Materials: The penumbra characteristics of the Elekta's Synergy‐S micro‐leaf beam modulator with 4mm leave width and BrainLab's Novalis μMLC with 3mm leaf width at isocenter were investigated. Kodak EDR2 film was exposed to approximately 200cGy for a variety of build‐up thicknesses (6MV photon beam, depth =1.5, 5, 10 and 15cm), field sizes (3cm2–10cm2), and treatment distances (90, 100, 105, 110, and 115cm SSD) for both square and circular fields. The penumbra information was expressed as 80% to 20% isodose distance. Results: Analysis of the measured penumbra revealed that BrainLab's μMLC had smaller effective penumbra (2.3mm–3.9mm at leaf‐side and 2.4mm–4.4mm at leaf‐end) as compared to that of Synergy‐S (3mm–6mm at leaf‐side and 4.3mm–7.8mm at leaf‐end) for square fields, at 100cm SAD. The average difference of effective penumbras at leaf‐side and at leaf‐end was about 50% for the Synergy‐S, while this difference was about 6% for BrainLab's μMLC. However, the effective penumbras for circular fields (3cm–9cm diameter) were comparable; ranging 4mm–7mm for the Synergy‐S μMLC and 3mm–6mm for the BrainLab's system. Discussion and Conclusions: Both the BrainLab's and the Elekta's μMLCs produce comparable effective penumbras for typical treatment conditions. The Synergy‐S does not use any adjustable jaw system. In this study, the adjustable jaws of BrainLab's μMLC were positioned at least 0.5cm behind the leaf‐ends to eliminate their effect on penumbra. The variation of penumbra on leaf‐end is significantly smaller for the BrainLab's μMLC. This difference could be due to several factors such as x‐ray target to leaf distance, target's shape and size, or the exact leaf tip design; these issues are being investigated. © 2007, American Association of Physicists in Medicine. All rights reserved.