Dual layer MLC dosimetric characteristics for small field and IMRT applications

Abstract

The purpose of this 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 while 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 on leaf stepping angle. The performance was studied with a triangular field, a cross leaf-end field and a circular field through two standard MLCs (Varian® MLC-80 and MLC-120), a dual-layer MLC (AccuKnife® DmMLC), and a cerrobend block. Measurements were made with 6 MV x-rays and EDR2 film at 5 cm depth in solid water at 100 cm SAD. 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 MLC-80, MLC-120 and DmMLC respectively. The planar dose difference between the MLC-80, MLC-120, and DmMLC against Cerrobend block was measured ranging +/-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 single layer of DmMLC, and at 2.4% in maximum and 2.1% in average when closing both layers. The MLC dependence of leaf stepping angle of the DmMLC ranges from 45⁰ to 90⁰, while standard MLC leaf stepping angle ranges from 0⁰ to 90⁰. In conclusion, the dose undulation, leaf-end transmission and MLC field dependence of the leaf stepping angle of the DmMLC were remarkably reduced compared to the standard MLCs. The DmMLC provided very comparable performance in field edge smoothing and in complex field shaping compared to Cerrobend block.