Purpose:The increasingly attractive stereotactic body radiation therapy(SBRT) for stage I lung cancer is accompanied by a large amount of monitor units(MU),leading to excessive out-of-field dose and prolonged beam-on time.The study aims to reduce the MU number in SBRT treatment for lung cancer by optimizing the planning parameters.Materials and Methods:Fourteen patients suffered from stage I Non-Small Cell Lung Cancer(NSCLC) were used in the study.Priority for the upper objective of the target(UOT),strength and Max MU setting in the MU objective function were adjusted to investigate their effect on MU number,organs at risk(OARs) sparing and beam-on time.Results:We found that the planning parameters influenced the MU number in a priority,strength and Max MU dependent manner.MU number with low,medium and high priority for the UOT was 443±25,324±55 and 260±28 MU/Gy,respectively.High priority for the UOT also spared the heart,spinal cord and lung while maintaining comparable target coverage than the low and medium priority group.MU number was continuously reduced with the strength setting increasing and Max MU setting decreasing.Combined with high priority for the UOT,maximum strength of 100 and 50％ Max MU setting in the MU objective function,the MU number could be reduced to as low as 228±22 MU/Gy without compromising the target coverage and OARs sparing.Beam-on time was proportional to MU number and the corresponding beam-on time could be shortened to 4.1±0.4 minutes.Conclusion:The priority for the UOT,MU constraining objective in the planning parameters impact on MU number in SBRT treatment for lung cancer.MU number can be reduced to 228±22 MU/Gy without compromising target coverage and OAR sparing by integrating high priority to the UOT,maximum strength and 50％ Max MU setting in the MU objective function."