Comparison of dosimetric characteristics of 120-leaf and 80-leaf multi-leaf collimators in a Varian linear accelerator for a 6-MV photon beam.

In this study, the dosimetric characteristics of multileaf collimators (MLCs) with 120 leaves and 80 leaves were evaluated. The dose rate, percentage depth dose, surface dose, dose in the build-up region, beam profile, flatness, symmetry, and penumbra width were measured by three field-defining methods: (1) "Jaw only", (2) "MLC only", and (3) "MLC + Jaw", for a 6-MV photon beam with the two MLC systems. Analysis of the dose rate showed that the dose rate for the "MLC only" field was higher than that for the other two fields. The surface dose was more pronounced for the "MLC only" fields. The dose in the build-up region was higher for the "MLC only" fields, and no significant difference was found in the percentage depth dose (PDD) beyond the dose maximum point (d(max)) for both MLC systems. Beam profiles showed that the differences in flatness and symmetry for both systems were less than 3%. The penumbra width between 80 and 20% isodose levels for the "MLC only" field was more than that for the other two field defining methods. The widths of the 50% dose levels of the beam profiles were analyzed. The dosimetric characteristics of the two MLC systems were the same in all aspects except the surface dose, penumbra, the dose in the build-up region, and the width of the 50% dose levels.