Objective: The incidence of multiple primary malignancies has increased in recent decades.Both metastases and primary cancers are common in lung.The present study attempts to analyze the results of multiple primary cancer (MPC) involving lung cancer by 18F-FDG PET/CT retrospectively.Additionally, analyze the clinical feature and imaging manifestation.As a result,summarize the diagnosis procedure of multiple primary cancer from above information.Method: According to Warren and Gates 3 standards of MPC, we collected 45 MPC patients involving lung cancer for retrospective analysis in our PET center from Sep 2004 to Dec 2009.The final diagnoses were all confirmed by pathology of biopsy or surgery.All the patients were scanned by Siemens Biograph Sensation 16 and Siemens Biograph 64HD PET/CT with 18F-FDG.All the patients took scans 1h after injection and delayed scans 2-3h after injection.The images were separately read by 2 experienced nuclear medical doctors.They described CT imaging manifestation and FDG metabolism.Lesions should be recorded in number, size, shape and FDG uptake, etc.Patients with lung cancer as first primary cancer were grouped in the First Primary Lung Cancer (FPLC).Patients with lung cancer as second primary cancer were grouped in the Second Primary Lung Cancer (SPLC).The data was statistically analyzed by Excel and SPSS15.0.Result: ①The incidence of MPC was 1.55%.②The most common second primary cancer was lung cancer in FPLC.The most common first cancer was breast cancer in SPLC.The most common pathology of multiple lung cancer was adenocarcinoma.③The male to female ratio was 1.25:1 in MPC.Gender was no significant difference in FPLC and SPLC (x2=0.54, P>0.05).④Age of onset of first primary cancer was 27~79 years old, and the average was 55.8 years old.Age of onset of second lung cancer was 40~84 years old, and the average was 62.1 years old.Both of them got to the peak in 60~70-year-old group.There was no significant difference of age between first primary cancer and second primary cancer in both FPLC and SPLC (t=1.985, 1.646,P>0.05).⑤11 patients (24.4%) were synchronous multiple primary cancer (SMPC).34 patients (75.6%) were metachronous multiple primary cancer (MMPC).The shortest interval of MMPC was 8 months.The longest was 31 years.The average was 76.1 months.The longest interval in FPLC was 372 months and the average was 76.6 months.The longest interval in SPLC was 144 months and the average was 29.7 months.There was significant difference between FPLC and SPLC (t=1.818,P=0.041<0.05).⑥45 MPC patients all took 71 PET/CT scan.1 first primary cancer was found.19 second primary cancers were detected, and 3 out of 19 were double primary lung cancer (DPLC).5 SMPC were diagnosed, including 2 DPLC.The rest 20 were follow-up after the treatment of MPC.⑦Second primary cancers were detected by PET/CT in 24 scans.19 out of 24 were confirmed MMPC in the follow-up of first primary cancer.16 patients (84.21%)had single lesions.They located in lung (13/16, 81.25%), pancreas (2/16, 12.5%), colon (1/16,6.25%).SUVmax of 15 second primary solitary pulmonary nodule was 1.3~22.9,the average was 6.5.⑧ CT imaging manifestation of the 22 FDG uptake focus included no malignant sign (1), one sign(7), two signs(8), three signs(3), four signs(1).Each lesion had 1.9 signs on average.Conclusion: In conclusion, we try to differentially diagnose MPC involving lung cancer from other lung tumor with the help of PET/CT.Our study shows the incidence of MPC is 1.55%.When lesions locate in uncommon or rare metastasis location, the possibility of MPC should be paid attention.Further imaging examination, serologic examination or other serum tumor markers should be taken to help diagnosis.PET/CT is valuable in detecting MPC including lung cancer.But the suspected lesions with FDG uptake should have biopsy or surgery for pathological results.Finally, we can make reasonable clinical treatment to improve survival time with PET/CT.