Any tumor is harmful to health, but the degree of harm is different. Accordingly, medical doctors distinguish tumors into two categories: benign tumors and malignant tumors. The tumor cells of benign tumors are close to the normal cells of corresponding tissues in form and function. The tumor mostly grows slowly and expansively, compressing the surrounding normal tissues, and can form envelope, so the demarcation is clear. The tumor grows locally and produces symptoms such as compression and obstruction, but the tumor cells do not shed and metastasize from the primary site to other sites and form new metastases. Therefore, most benign tumors can be completely removed without recurrence and can be completely cured, which is less harmful to human body. 2. The structure and function of malignant tumor tumor cells differ greatly from corresponding normal cells, with strange morphology and weakened, enhanced or lost functions. The tumor grows fast and often invades the surrounding normal tissues with unclear demarcation. Tumor cells are easily detached from the tumor and run through lymphatic vessels, blood vessels or other cavities to form new metastases elsewhere. In addition to causing symptoms of pressure and obstruction, malignant tumors may also combine with bleeding and necrosis, fever, etc. Many patients with malignant tumors, especially in the advanced stage of the disease, can be extremely emaciated called cachexia. As malignant tumor is infiltrative growth, it is difficult to be completely removed and easy to recur after surgery. Moreover, tumor often metastasizes to local lymph nodes or spreads to the whole body, which is difficult to be completely cured and can eventually lead to death of patients. It is worth noting that sometimes there is no absolute boundary between benign and malignant tumors, and some tumors can be in between, called junctional tumors. For example, papillary tumors of the bladder have a benign cellular pattern, but are prone to recurrence and even turn malignant. Benign tumors such as hemangiomas are mostly without envelope and poorly defined, and are prone to recurrence after resection. For example, benign intracranial tumors (meningioma and astrocytoma) can compress the brain tissue and block the ventricular system, leading to great harm; for example, mesothelioma in the heart, which is only a few millimeters in size, can cause heart rhythm disturbance and lead to sudden death. Malignant tumors do not have poor prognosis. For example, basal cell carcinoma of the skin grows slowly, hardly metastasizes and can be completely cured after treatment. Some benign tumors can turn into malignant tumors if not treated in time, for example, ovarian tumors can turn into ovarian cancer. Occasionally, malignant tumors can also be transformed into benign tumors, for example, neuroblastoma, a malignant tumor in children, can be transformed into benign ganglioneuroma.