Objective To analyze the relevant factors influencing 18F-FDG uptake in the primary lesion of breast invasive ductal carcinoma (BIDC).Methods A total of 160 female patients underwent 18 F-FDG PET/CT examination from 2010 to 2015 and breast lesions were revealed.Lesions were divided into benign group (n =118) and malignant group (n =49,BIDC) according to pathological results.KruskalWallis H test and Mann-Whitney u test were performed to compare SUVmax of the two groups,and to investigate the relationship between the SUVmax of breast malignant lesion and patients' age as well as clinical pathological parameters including T stage,lymphatic vessel invasion,nuclear grade,route of metastasis,ER,PR,HER2 and Ki-67 expression,and subtype of breast cancer.The diagnostic efficiency of 18F-FDG PET/ CT in differentiating benign and malignant breast lesions was analyzed using ROC curve analysis.Results The SUVmax of BIDC was 6.09(3.88,9.26),higher than that of breast benign lesion (1.35 (0.95,2.35);u=341.0,P＜0.05).The SUVmax of BIDC showed statistically significant difference between groups with different T stage,with or without lymphatic vessel invasion,with different nuclear grade,different routes of metastasis and different Ki-67 expression (u:117.5-209.5,H=7.70,P＜0.01 or 0.05).For all breast lesions,lesions with the maximum diameter ≤ 2.0 cm and lesions with the maximum diameter ＞2.0 cm,the optimum cutoff values of SUVmax were ＞2.60,＞ 1.71 and ＞3.97,respectively.When the optimum cutoff values of SUVmax for breast lesions with the maximum diameter ≤2.0 cm were selected as ＞ 1.71 and ＞2.60,the Youden indexes were 0.66 and 0.61(z=0.566,P＞0.05).When the optimum cutoff values of SUVmax for breast lesions with the maximum diamter ＞2.0 cm were selected as ＞3.97 and ＞2.60,the Youden indexes were 0.89 and 0.81(z=0.748,P＞0.05).Conclusions T stage,lymphatic vessel invasion,nuclear grade,route of metastasis and Ki-67 expression of BIDC influence the uptake of 18F-FDG by tumor tissues.The SUVmax of the primary lesion of BIDC is related to the size of lesion,and thus the diagnostic threshold of SUVmax should be decreased appropriately for small lesions.