Metastasis is one of the important features of malignant tumors, which refers to the spread of tumor cells from the primary site to other parts of the body and proliferate again to become a tumor lesion, and this lesion is also called metastasis. Generally speaking, the occurrence of metastasis often indicates that the tumor has entered a late stage, which will bring certain difficulties and challenges to the treatment. So how do tumors metastasize? What are the characteristics of each type of metastasis? There are several ways of tumor metastasis, such as direct infiltration, bloodstream metastasis, lymphatic metastasis and implantation metastasis: 1. Direct infiltration Direct infiltration refers to the direct invasion of tumor cells into the surrounding adjacent tissues, which causes the increase and spread of tumor. Among several tumor metastasis pathways, direct infiltration is often the most predictable because the tumor only involves the surrounding tissues. Although mostly expected, direct invasion can also cause quite troublesome consequences, such as direct invasion of bladder by rectal cancer, localized invasion of esophagus by lung cancer and so on. Such cases are often difficult to operate or require combined organ resection, thus the assessment of local tissue involvement is also very important in clinical work. 2.Bloodstream metastasis As we all know, there are intricate blood circulation systems all over the human body, which is like a highway in all directions, transporting nutrition and oxygen to every part of the body continuously. When tumor cells invade into such blood vessels, they will quickly transfer to organs far away from the primary foci with blood circulation, and then take root to create new tumor metastases, which will cause corresponding harm. Since tumor cells in hematogenous metastasis often spread along the direction of blood flow, just like from one stop to the next on the highway, the site of hematogenous metastasis of each organ tumor is largely predictable, but due to the complexity of the circulatory system, there may be difficult to count the country roads between the highways, so the prediction difficulty of hematogenous metastasis is much higher than that of local infiltration. 3. Lymphatic metastases Many people may have heard of the term “lymphatic system”, but have little understanding of its true meaning. In fact, in addition to the blood circulation system consisting of heart and blood vessels, there is also a set of lymphatic system consisting of lymph nodes and lymphatic vessels, which mainly plays the role of immune defense. Similar to bloodstream metastasis, tumor cells can also metastasize by entering the lymphatic circulation, which is actually more common in clinical practice, mainly manifesting as the involvement of regional lymph nodes and occasionally the metastasis of distant lymphatic structures. 4. Implantation metastasis As the name implies, implantation metastasis refers to the ability of certain tumor cells to take root wherever they are sown like seeds, forming a metastatic lesion. This kind of metastasis mainly occurs in tumors in the thoracic and abdominal cavities, which will break through the organs and spread in the pleura, peritoneum, pericardium, etc., and often form a diffuse metastasis like “starry sky”. Although the number of such diffuse metastatic lesions is large, each of them may be small in size, so they may be undetectable in the imaging examination and only be seen during laparoscopy or open surgery, which is also a difficult problem in clinical practice. In summary, if we compare tumor metastasis to a war, then direct infiltration is undoubtedly a frontal battlefield step by step, advancing gradually, one step at a time, and this kind of battlefield is basically predictable; while hematologic and lymphatic metastasis is like a long-distance run by railroad, although the number of transported tumor cells is more limited, but it will often invade to a distant and unpredictable location. Sometimes, they will land in a fixed location and only involve a specific distant organ; sometimes, they will blossom in the sky and cause diffuse invasion in the body cavity, thus making the operation impossible. In fact, the metastatic site and metastatic mode of a tumor are determined by its own nature, and once metastasis occurs, the prognosis of patients is often poor. However, we believe that with the gradual understanding of metastasis mechanism and the continuous progress of medical technology, the treatment for metastatic tumors will be increasingly improved, and we will eventually win this war of metastasis.