Breast cancer in situ is not a precursor of breast cancer. Breast cancer in situ is one of the pathological types of breast cancer, which indicates that it is already in the early stage of breast cancer. Breast cancer can be categorized into non-invasive cancer and invasive cancer, and non-invasive cancer includes intraductal carcinoma, lobular carcinoma in situ and nipple eczema-like breast cancer, among which intraductal carcinoma and lobular carcinoma in situ are breast cancer in situ. Common symptoms of carcinoma in situ are painless breast lumps, accompanied by skin edema and nipple discharge in some patients. Carcinoma in situ means that the cancer is confined to the epithelial layer of the inner mucosa and has not broken through the basement membrane, which is in early stage and has a better prognosis. If it is not detected and treated in time, it may develop into invasive cancer. Therefore, if diagnosed as carcinoma in situ, it is already in the early stage of breast cancer and should be treated as early as possible to avoid aggravation of the condition.