Secondary malignant tumor, also called metastatic malignant tumor, is relative to primary malignant tumor, which refers to a malignant tumor of an organ tissue that is not primary to that organ tissue, but is metastasized from malignant tumor of other organ tissues. To use a simple analogy, what is usually referred to as breast cancer is a malignant tumor that originates from breast tissue, but when breast cancer metastasizes to the brain, this malignant tumor of the brain can be called a secondary malignant tumor. Generally speaking, secondary malignant tumor is the same as the primary malignant tumor from which it originated in terms of pathological histological features and growth characteristics. Clinically, many patients with tumors cannot be diagnosed because it is difficult to obtain biopsies from the primary site for pathological examination, but the diagnosis is eventually established by obtaining secondary tumor tissues for pathological examination.