The most common way for cancer to spread is through the lymphatic system. The lymph system has its own channels that circulate throughout the body, similar to the veins and arteries of the bloodstream. These channel are very small and carry a tissue fluid called lymph throughout the body. Often when a solid tumor is removed by surgery, the surgeon will remove not only the tumor but the neighboring lymph glands, even though there is no visible sign of cancer in those glands. This is done as a precautionary measure, because if even one cell has broken away from the tumor and lodged in the lymphatic system, the cancer could continue growing and metastasizing.
Cancer can also metastasize through the bloodstream.
Cancer cells, like healthy cells, must have a blood supply in order to live, so all cancer cells have access to the bloodstream. Malignant cells can break off from the tumor and travel through the bloodstream until they find a suitable place to start forming a new tumor. Sarcomas spread through the bloodstream, as do certain types of carcinomas, like carcinoma of the kidneys, testicular carcinoma, and Wilms' tumor, a type of kidney cancer seen in young children. Cancers may spread by more than one route. Some cancers that spread this way do not venture very far from the original site.