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

Abstract: The aim of this study is to evaluate the effect on the MLC-based small beam dose distribution according to the different specific dosimetric leaf gaps. The dosimetric static leaf gap was determined by comparing, the profiles of the MLC-based small beam with those of the collimated small fields (Square fields of 1, 2, 3, and 4 cm). The results showed that an approximately 2 ㎜ gap was optimal to the Millennium 120 leaf MLC of a Varian 21EX 6 MV photon beam. This study investigated how much the leaf gap affects the planning results and the actual dose distribution. The doughnut shaped PTV (6.1 cm3) and the inner OAR (0.3 cm3) were delineated for delicate IMRS test planning. The applied leaf gaps were 0, 1, and 2 mm for comparison. The measured dose distributions were compared with the dose distribution in the RTP system. There were the maximum dose differences at inside PTV, outside PTV, and inner OAR of 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 respectively. In a human head phantom (model 605, CIRS, Norfolk, Virginia) study, there were large dose differences of 1.3 to 12.7% for the measurements with the MLC files generated by three different leaf gaps. In addition, the planned results were similar, while the measurements showed a large dose difference according to the leaf gaps. It strongly represents that the planned results using a commercial inverse planning system, which was commissioned by the general collimated field data, could have the probable discrepancies between the RTP and measured results. Key words: MLC-based small beam, dosimetric static leaf gaps PACS number: 87.53.Dq