you can die if a hugh tumor in your lung break down into thousand smaller pieces and the blood that supply the tumor flood your lung with a enormous amount of blood that prevent you from breathing, or a tumor compress your brain and 'shut down' the center that control your heart beat or breathing and your heart or your lung stop working immediately. Or if there are so many tumor in your body that take all the nutrient that your body need, you may die because there are not enough energy for the normal cell in your body work properly, especially brain cell. So to concluded, you can't die because of cancer, you die because of it's complication