Carcinoma in situ and invasive ductal carcinoma are both pathologic types of breast cancer, with invasive ductal carcinoma being more serious. Carcinoma in situ means that the cancer cells are confined within the basement membrane without infiltrating into the interstitium or lymphatic vessels or blood vessels. Invasive ductal carcinoma refers to the development of ductal carcinoma in situ, in which the cancer cells break through the ductal basement membrane and infiltrate into the interstitium. Ductal carcinoma in situ can be clinically cured and has a good prognosis, while invasive ductal carcinoma has a small possibility of being clinically cured and has a worse prognosis than carcinoma in situ. The etiology of breast cancer is still unclear, and it is difficult to propose exact etiologic prevention, but emphasizing on early detection of breast cancer and detecting cases through screening will improve the survival rate of breast cancer patients. In order to detect malignant tumors in time, women are advised to have regular medical checkups so as not to delay their condition.