When cancer spreads, there are two main pathways of escape from the organ where the cancer started. The first one is the lymphatic system. Patients have commonly heard of cancer spreading to lymph nodes. The other way is through blood vessels and into the bloodstream, where they may set up sites of deposits of cancer spread far away. For example, the lungs or the bones. But kidney cancer has one very unique, potential complication. It’s unlike most other cancers. It likes to grow inside what we call the lumen or the inside blood flow portion of the veins that drain blood from the kidney. An actual tongue or finger projection of the cancer itself called a tumor thrombus starts to grow inside the kidney vein and it crawls towards the heart. It makes its way from the kidney vein to the largest vein in the body called the inferior vena cava. And if it keeps growing, can make its way all the way to the interior of the heart. This propensity for the kidney cancer to grow inside of veins draining from the kidney is one of the things that makes it unique. If this causes symptoms, because it’s blocking blood flow from other parts of the body as well, people can get leg swelling. It’s one of the features that makes kidney cancer unique. In patients whose kidney tumors form a thrombus that crawls its way through the veins, draining from the kidney and into the heart. This can slow the blood returning to the heart from other parts of the body and cause symptoms like leg swelling. It’s this feature which does not occur in all kidney cancers that makes kidney cancer unique.