Study of dosimetric penumbra due to multileaf collimation on a medical linear accelerator.

Abstract

Investigations of dosimetric penumbra produced by multileaf collimation on a medical linear accelerator are presented. Multileaf collimators (MLCs) can be designed with at least three different shaped leaf-end profiles: straight, divergent, and curved. Assessment of the dosimetric effects of the collimator edge profiles was implemented using a fast Fourier transform (FFT) convolution algorithm. Accelerator source intensity was considered to have a Gaussian distribution. The calculated dose profile, for a source-to-surface distance of 100 cm and at depth of 10 cm in a water phantom, was fitted to a penumbral-forming function from which the penumbral width between 80% and 20% of the central axis dose was obtained. Calculation performed at various field sizes showed that curved collimator leaf-end produces a wider penumbra than the diverging collimator leaf-side. Film measurements agreed with the calculations within an uncertainty of less than 2 mm. The effect of backup jaws for the MLC and of the lower pair of diverging diaphragms on dosimetric penumbra was also investigated. This study is useful for characterizing collimator edge effects and for optimizing new collimator designs.