Metastatis means little cancer "seed" cells are floating around all over your bloodstream, right? This means that at any random time, a cancerous growth can suddenly pop up in any random region with no warning.

In particular, metastasis is most common in the liver and lung. Your lungs receive as much bloodflow as the rest of your body put together and have lots of small blood vessels for gas exchange, so it's easy for circulating cells to get strained out and start growing there. And your liver similarly gets all the blood from your digestive tract passing through it before it returns to your heart, moving it through lots of low-velocity low-pressure capillary passages where liver cells perform lots of chemical reactions and free cells have an easy time settling down. Unfortunately both your liver and lungs are extremely important organs.