The lump in the opposite breast after unilateral breast cancer surgery can be malignant (breast cancer), benign tumor or adenopathy, etc. For malignant cases, it is necessary to distinguish whether it is recurrent metastasis of the first side or primary cancer of the second side, because the treatment principles of the two are different, so the differential diagnosis is more important, based on the time of occurrence, lesion location, growth pattern, etc., which helps to distinguish. is the most reliable basis. Currently, it is believed that the probability of developing cancer in the contralateral breast is increased after breast cancer on one side, which is 5-11 times higher than that in the normal breast according to foreign reports. The risk factors for the occurrence of contralateral breast cancer are related to multicentric cancer in the first breast, family history of breast cancer (BRCA mutation), and the age of onset of the first cancer (<50 years old). Prevention of contralateral breast cancer after unilateral breast cancer surgery: 1. Primary prevention: Pay attention to standardized chemotherapy and endocrine therapy after the first breast cancer. Triamcinolone acetonide can prevent the occurrence of contralateral cancer while treating breast cancer, and its use for 5 years can reduce the risk of contralateral breast cancer by 47%. In the United States, preventive contralateral mastectomy has become a treatment option for breast cancer patients. Lobular carcinoma, multicentric carcinoma, family history of breast cancer, and BRCA gene mutation are the main reasons for performing preventive contralateral mastectomy, and preventive mastectomy has a protective effect on these high-risk women and can reduce the incidence of breast cancer by more than 90%. 2.Secondary prevention: Early detection and early diagnosis and treatment. Clinically, after unilateral breast cancer surgery, lifelong review is required, and special attention should be paid to the examination of the contralateral breast in the first five years. For patients with family history of breast cancer, BRCA gene mutation and young unilateral breast cancer, clinical follow-up and patient education should be strengthened to strive for early detection of contralateral breast cancer and early formal treatment.