Cancer-associated thrombosis

Cancer-associated thrombosis is a blood clot that forms inside a vein or an artery, and it's the cancer that makes that clot more likely to form. That's exactly what thrombosis means: blood clotting inside the very vessel it should be flowing through as a liquid. Sometimes that clot is the first sign of the cancer. It shows up while the tumor is still hidden and nobody knows it's there.

Here's what happens inside. To form a clot, blood relies on the coagulation cascade, a chain of steps where each one sets off the next, like a row of dominoes. With cancer, the blood is in a state of hypercoagulability, which means it clots more easily than normal. Malignant cells, the cells that make up the cancer, produce tissue factor, a substance that seems to give the push that knocks the row over until it reaches factor Xa, one of the dominoes in that cascade. Some cancer cells take an even more direct route: they make cancer procoagulant, which leads directly to factor Xa, as if someone tipped over that one domino without waiting for the ones before it to fall.

The clot most often forms in the veins. It can be a deep vein thrombosis, a clot in the deep veins, or a pulmonary embolism, when a clot blocks an artery in the lung. Less often, it forms in the arteries, and the result is a stroke, where part of the brain loses its blood supply, or a myocardial infarction, where part of the heart loses its blood supply.