Sometimes adults get blood clots in their veins, a condition known as deep venous thrombosis (DVT). The blood clots usually form in leg veins, but they can occur in the veins of the arms and shoulders, particularly in people who have cancer and a central venous catheter (a tube inserted through the skin into a vein near the heart to deliver drugs such as those used in chemotherapy). Blood clots in the deep veins are dangerous because they can break free and travel to the lungs. This problem, called pulmonary embolism (clot in the lung), can be fatal. It is important to diagnose DVT because special blood-thinning drugs reduce the risk that a clot will travel to the lungs.