 Estimated 3 year disease free survival for the treatment group and the observation group is 66 and 54, respectively. The 3 year disease free survival rate for the observation group was estimated based on retrospective multicenter study on 1132 stage 1 NSCLC patients, previously reported in Korea. During the 3 year of follow up duration, the withdrawal rate was presumed to be 10 because of loss of follow-up, violation of the consent and other causes. Study population enrollment is estimated as 48 months and mean follow-up duration is estimated as 24 months. Total duration of the study is set as 72 months. In this study design, the number of the high risk patients of each group is 253 and it supports 80 power of test with a two-sided alpha level of 0.05. 