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Jin et al, in an intensity-modulated radiotherapy and radiosurgery study of 54 patients, concluded that the 3 mm MLC has a better conformity index and better sparing of small organs at risk (OARs) than either the 5 mm or the 10 mm MLC with a target volume dependence. However, the authors concluded that quantitative differences between the 3 and 5 mm leaf MLC (based on 5% for tissue sparing) may not be clinically significant for some cases. Monk et al, in a study of 14 intracranial cases, showed that 3 mm MLC improves both planning target volume (PTV) conformity and normal tissue sparing over 5 mm MLC for intracranial static-field SRS. Subsequently, Fiveash et al compared intensity-modulated radiotherapy plans between a 5 mm MLC and a 10 mm MLC in three cranial cases and observed noticeably better sparing of optic structures for the 5 mm MLC. The authors showed that the 1.7 and 3.0 mm MLCs met the Radiation Therapy Oncology Group guidelines for static-field SRS treatment planning. Kubo et al were the first to assess the conformity of three-dimensional (3D) conformal plans using 1.7, 3 and 10 mm leaf width MLC systems. The advantage of the smaller leaf width has been studied by several groups, but with mixed results.
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However, over the last 15 years, multileaf collimators (MLCs), now a routine appendage to modern linear accelerators (linacs), have evolved in terms of both field size and width of the individual tungsten leaves, and it is intuitive to assume that target dose conformity and/or the steepness of the dose gradient can be influenced by decreasing MLC leaf width. Historically, linear accelerator-based SRS treatment planning and delivery has relied upon non-coplanar arc therapy delivered through small (≤40 mm) circular collimators. The principal goal of stereotactic radiosurgery (SRS) is to provide a method for focal irradiation of target tissue to high doses without increasing normal tissue complication.
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