Abstract： Objective To discuss the value of dual-time-point 18FDG PET-CT imaging on involved field radiotherapy for hilar and mediastinal metastatic lymph nodes in patients with non-small cell lung cancer (NSCLC).Methods Fifty-four patients with NSCLC were included in this analysis,including 34 men and 20 women with mean age of 59(34-76)years.Two sequential PET-CT scans given 3-5 days before surgery were standard single-time-point imaging for the whole body and delayed imaging for the thorax.The pathologic data were used as golden standard to determine the difference between the standard single-time-point and dual-time-point FET-CT imaging in the definition of gross target volume(GTV)of involved-field radiotherapy for metastatic lymph nodes. Results For hilar metastatic lymph nodes,the GTV defined by single-time-point imaging was consistent with pathologic GTV in 21 patients(39%),comparing with 31 patients(57%) by dual-time-point imaging.Using pathologic data as golden standard,GTV alteration defined by single-time-point imaging had statisticaly significant difference comparing with that defined by dual-time-point imaging(u=519.00,P=0.023).For mediastinal metastatic lymph nodes,the GTV defined by single-time-point imaging was consistent with pathologic GTV in 30 patients(56%),comparing with 36 patients(67%)by dual-time-point imaging.Using pathologic data as golden standard.GTV alteration defined by single-time-point imaging had no statisticaly significant difference comparing with that defined by dual-time-point imaging(u=397.50,P=0.616).Conclusions For patients with NSCLC receiving involved-field radiotherapy,GTV definition for hilar and mediastinal metastatic lymph nodes by dual-time-point imaging is more consistent with that by pathologic data.Dual-time-point imaging has a larger value in terms of target delineation for hilar and mediastinal metastatic lymph nodes.