Effects of static dosimetric leaf gap on MLC‐based small‐beam dose distribution for intensity‐modulated radiosurgery

The aim of the present study was to evaluate the effect of various specific dosimetric leaf gaps on the multileaf collimator (MLC)-based small-beam dose distribution. The dosimetric static leaf gap was determined by comparing the profiles of small MLC-based beams with those of small collimated fields (square fields of 1, 2, 3, and 4 cm). The results showed that an approximately 2-mm gap was optimal with the Millennium 120-leaf MLC (Varian Medical Systems, Palo Alto, CA) and a Varian 21EX 6-MV photon beam. We also investigated how much the leaf gap affects the planning results and the actual dose distribution. A doughnut-shaped planning target volume (PTV, 6.1 cm3) and inner organ at risk (OAR, 0.3 cm3) were delineated for delicate intensity-modulated radiosurgery test planning. The applied leaf gaps were 0, 1, and 2 mm. The measured dose distributions were compared with the dose distribution in the treatment planning system. The maximum dose differences at inside PTV, outside PTV, and inner OAR were, respectively, 22.3%, 20.2%, and 35.2% for the 0-mm leaf gap; 17.8%, 22.8%, and 30.8% for the 1-mm leaf gap; and 5.5%, 8.5%, and 6.3% for the 2-mm leaf gap. In a human head phantom (model 605: CIRS, Norfolk, VA) study, large dose differences of 1.3%-12.7% were noted for the measurements made using the MLC files generated by the three different leaf gaps. The planned results were similar, and measurements showed a large dose difference associated with the various leaf gaps. These results strongly suggest that plans generated by a commercial inverse planning system commissioned using general collimated field data will probably demonstrate discrepancies between the planned treatments and the measured results.