Dosimetric characteristics of dual-layer multileaf collimation for small-field and intensity-modulated radiation therapy applications

The purpose of the present work was to measure the performance characteristics in the penumbra region and on the leaf-end of an innovative dual-layer micro multileaf collimator (DmMLC). The DmMLC consists of two orthogonal (upper and lower) layers of leaves; a standard MLC consists of one layer. The DmMLC provides unique performance characteristics in smoothing dose undulation, reducing leaf-end transmission, and reducing MLC field dependence of the leaf stepping angle. Two standard MLCs (80-leaf and 120-leaf versions: Varian Medical Systems, Palo Alto, CA), a DmMLC (AccuKnife: Initia Medical Technology, Canton, MA), and a Cerrobend (Cerro Metal Products, Bellefonte, PA) block were used in performance studies involving a triangular field, a cross leaf-end field, and a circular field. Measurements were made with 6-MV X-rays and extended dose range film at a depth of 5 cm in Solid Water (Gammex rmi, Middleton, WI) at a source-axis distance of 100 cm. The field penumbra width measured between the 20% and 80% isodose lines through the MLC-80, MLC- 120, DmMLC, and Cerrobend block were 9.0, 5.0, 3.0, and 2.0 mm respectively. The dose undulation amplitude of the 50% isodose line was measured as 5.5, 2.0, and 0.5 mm for the MLC-80, MLC-120, and DmMLC respectively. The planar dose difference between the MLC-80, MLC-120, and DmMLC against Cerrobend block was measured as ranging at ±52.5%, ±35.0%, and ±20.0% respectively. The leaf-end transmission was measured at 22.4% in maximum and 15.4% in average when closing a single layer of the DmMLC, and at 2.4% in maximum and 2.1% in average when closing both layers. The MLC dependence of the leaf stepping angle with the DmMLC ranged from 45 degrees to 90 degrees. The standard MLC leaf stepping angle ranged from 0 degrees to 90 degrees. In conclusion, the dose undulation, leaf-end transmission, and MLC field dependence of the leaf stepping angle with the DmMLC were remarkably reduced as compared with those of the standard MLCs. And as compared with Cerrobend block, the DmMLC provided very comparable performance in field-edge smoothing and in the shaping of complex fields.