Dosimetric characteristics of a double-focused miniature multileaf collimator

The dosimetric characteristics of a double-focused miniature multileaf coll imator (mMLC) attached to a Philips SL75/5 linear accelerator (linac) have been investigated. Output factors, percentage depth-dose, penumbra, leaf tr ansmission, and leakage between the leaves were measured for the 6 MV x-ray beam on this accelerator. Because leakage both through and between the lea ves is minimal, the linac jaws can be kept fixed while the mMLC leaf config uration is modified for different aperture shapes. This allows for accurate output prediction using the equivalent square formalism. Percent depth-dos e measured for fields defined by the mMLC show little deviation from the pe rcent depth-dose measured for fields defined by the machine jaws or Lipowit z metal blacks. Because the mMLC matches beam divergence in both directions , allows minimal beam transmission, and has a large source-to-collimator di stance, the;penumbra is sharper for fields defined by the mMLC than for fie lds defined by the linac jaws or Lipowitz metal blocks. Based on these data , dose calculations for mMLC-defined fields can be applied with no change i n procedures from those used for fields defined using conventional methods. (C) 1999 American Association of Physicists in Medicine. [S0094-2405(99)00 905-0].