J Sun1, Y Zhu. 1. Department of Clinical Physics, Ontario Cancer Institute, Toronto, Canada.
Abstract
PURPOSE: Investigations of dosimetric penumbra produced by multileaf collimation on a medical linear accelerator are presented. METHODS AND MATERIALS: 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. RESULTS: 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. CONCLUSIONS: This study is useful for characterizing collimator edge effects and for optimizing new collimator designs.
PURPOSE: Investigations of dosimetric penumbra produced by multileaf collimation on a medical linear accelerator are presented. METHODS AND MATERIALS: 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. RESULTS: 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. CONCLUSIONS: This study is useful for characterizing collimator edge effects and for optimizing new collimator designs.