Objective:To investigate the changes of serum Golgi protein 73 (GP73) expression and its clinical significance in breast cancer patients before and after radiotherapy.Methods:A total of 135 patients with primary breast cancer who were diagnosed and treated in Qingdao Hospital Affiliated to Shandong First Medical University Hospital from Aug. 2016 to Dec. 2017 were selected and received standard radiotherapy after surgery. Enzyme-linked immunosorbent assay (ELISA) was used to determine the content of GP73 in the serum of patients before and after radiotherapy, and the differences in the expression levels of GP73 before and after radiotherapy in patients with different molecular phenotypes, tumor stages, and pathological types were compared. Association between the expression of serum GP73 and clinical outcome before and after radiotherapy was analyzed.Results:The expression of GP73 in breast cancer patients was (117.69±33.57) mg/L before radiotherapy and (101.88±30.92) mg/L after radiotherapy, and the difference was statistically significant ( t=4.025, P<0.001) . Different molecular phenotypes were stratified and found that the expression of serum GP73 in patients with luminal A, luminal B, HER-2 overexpression, and triple negative after radiotherapy decreased compared with those before radiotherapy, but only the HER-2 overexpression type had statistically significant difference between before and after radiotherapy ( P<0.05) . Stratification according to different tumor stages showed that the expression of serum GP73 in patients with stage I, II, III and IV after radiotherapy was lower than that before radiotherapy, but only the patients with stage II, stage III, and stage IV were compared before and after radiotherapy, and the difference was statistically significant ( P<0.05) . Stratification according to different pathological types showed that the expression of serum GP73 in patients with invasive ductal carcinoma before and after radiotherapy was significantly lower in patients with invasive lobular carcinoma ( P<0.05) . In addition, the 1-year survival rate of the descending group was significantly higher than that of the ascending group, while the local recurrence rate and the occurrence of distant metastasis were significantly lower than those of the ascending group, and the difference was statistically significant ( P<0.05) . Conclusions:The expression level of serum GP73 in breast cancer patients after radiotherapy is significantly higher than that before radiotherapy, and it has a certain relationship with molecular phenotype, tumor stage, and pathological type. At the same time, the increase in serum GP73 expression after radiotherapy may be detrimental to the clinical outcome of patients.