Improving the resolution of intensity modulated delivery by reducing the multileaf collimator sampling distance

The conformality of a dose distribution delivered by a multileaf collimator (MLC) for intensity modulated radiation therapy (IMRT) is limited in the direction perpendicular to leaf motion by the finite leaf width. Two methods of improving the resolution of IMRT intensity maps in this direction were investigated. In the first, the desired fluence distribution is considered to be sampled by the MLC, with the sampling distance being the center-to-center distance between the MLC leaves. The sampling distance is reduced below the leaf width by combining separate irradiations with a couch shift between them. This has been applied to static field therapy [Galvin et al., Int. J. Radiat. Oncol., Biol., Phys. 35, 89–94 (1996)], and was proposed for IMRT by Bortfeld et al. [Med. Phys. 27, 2494–2502 (2000)]. In the second method, two MLC component fluences, with leaf width L = 2Δy and offset by Δy, are combined to reproduce desired intensity bins with Δy width. The effect of MLC leaf sampling distance on dose resolution was quantified for both 1.0 and 0.5 cm MLC leaf widths, utilizing a high resolution bar-pattern fluence, an annular shaped fluence, and an intensity step-edge. Improvement in resolution was found for the 1.0 cm leaf width at a sampling distance of 0.5 cm, with only a small benefit for further reduction. For the 0.5 cm leaf width, a sampling distance of 0.25 cm resulted in a dose resolution that was nearly independent of direction.