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Purpose Leaf transmission is an important characteristic of multileaf collimator(MLC).The purpose of this work was to evaluate its dosimetric impact on intensity-modulated radiotherapy(IMRT)treatment plan of advanced lung cancer patients.Methods Five MLCs with different leaf transmissions(0.01%,0.5%,1.2%,1.8%and 3%)were configured for a Varian accelerator in Pinnacle 3 treatment planning system.Correspondingly,five treatment plans in the same optimization setting were created for a patient.Total 11 patients suffered from advanced lung cancer were investigated in this study.The plan quality and variations were evaluated quantitatively by several dosimetric indexes.Results All five plans for each patient met the dose requirement for the PTVs and had the similar target dose homogeneity(p > 0.05).All plans have SD value more than 0 and less than 3.5.Among 11 patients,6 patients have SD values more than 1 and less large one than 2.5 for all five plans.Two patients had SD values less than 1 for all five plans.The rest three patients had SD value more than 0.5 and less than 1.5 for all five plans.For larger value of MLC transmission,the overall SD values were increased and the plan dose requirement was violated more extensively.The total SD without NT showed the similar result as total SD with NT.Besides,the five plans with different MLC transmission show significant statically differences(p < 0.05).Thus the large MLC deviation value could result in the deviation of plan quality.Conclusions None of the plans met the pre-defined dose requirements since all patients selected were in were in late stage and hardly controlled locally even with the perfect MLC transmission model.The clinical requirements were violated increasingly as MLC transmission became large.Therefore the decrease of MLC transmission did has certain effect and it improved target coverage and reduced normal tissue radiation.To achieve the high quality plans and benefit the patient health care,it is worth of designing MLC with less transmission.