Comparison of a multi-leaf collimator with conformal blocks for the delivery of stereotactically guided conformal radiotherapy

Stereotactically-guided conformal radiotherapy is a practical technique for irradiating irregular lesions in the brain. The shaping of the conformal f ields may be achieved using lead alloy blocks, a conventional multi-leaf co llimator (MLC) or a mini/micro-MLC. Although the former gives more precise shaping, it is labour intensive. The latter methods are more practical as b oth mould room and treatment room times are reduced, hut the shaping is lim ited by the finite leaf-width. This study compares treatment plans, in term s of normal tissue doses and tumour coverage, for fields shaped using confo rmal blocks and a conventional MLC in two series of geometrical shapes and nine patient rumours. For the range cif tumour sizes considered (volumes 14 -264 cm(3), minimum dimension 30 mm, maximum 102 mm), the MLC treats, on av erage, 14% (range 3-34%) and 17% (range 0-36%) more normal brain tissue tha n conformal blocks to >50% and >80% of the prescription dose, respectively. The large variability is due to strong dependence on tumour shape and the presence of partial leaf-widths in the MLC fit. It is therefore important t o consider both of these effects when deciding whether the MLC is appropria te for a particular target volume. (C) 1999 Elsevier Science Ireland Ltd. A ll rights reserved.