Objective To analyze the application value of tumor/breast volume ratio measured by magnetic resonance imaging（MRI） in operative treatment of breast cancer. Methods Eighty-six breast cancer patients who underwent breast-conserving surgery in a hospital in Liaoyang City from February 2018 to April 2020 were selected for the study

and those who underwent MRI examination before surgery served as the observation group（42 cases

47 lesions）

and those who did not undergo MRI examination were considered as the control group（44 cases

45 lesions）. The clinical data

positive margin rate

rate of changes in surgical plan and reoperations of the two groups were compared

and the predictive value of tumor/breast volume ratio on positive surgical margin was analyzed. Results The difference between the two groups was not statistically significant（P>0.05） when comparing the positive rate of lesion margin

the rate of changes in surgical plan and the rate of reoperation. The tumor/breast volume ratio of patients with positive lesion margins in the observation group was greater than that of those with negative margins

and the difference was statistically significant（P<0.05）. The results of subject operating characteristic（ROC） analysis showed that the area under the curve（AUC） of the tumor/breast volume ratio for predicting positive lesion margins was 0.819（95% CI:0.718-0.925

P<0.001）; the optimal cut-off value of the tumor/breast volume ratio was 6.07%

with a sensitivity of 79.8% and specificity of 77.5% for predicting positive margins. Conclusion Preoperative MRI examination does not affect the rates of changes in surgical plan

reoperation and positive margin of lesion in breast cancer patients

but measuring the tumor/mammary volume ratio can effectively predict positive margin and provide a basis for clinical selection of the most appropriate type of surgery.