Chances of getting cancer? I'm 20. Chances of getting cancer or specifically brain cancer? Over a lifetime? How to prevent? Thanks

Good odds bad diseas. Fortunately Incidence of brain cancer is less than 20 cases per 100,000 population unfortunately most of the brain tumors are malignant. You can't prevent it ( as you did not chose your genes ) Don't worry about it enjoy your life , you will be fine.
Life's dangerous. Assuming you don't have one of the genetic cancer family syndromes, your odds of a primary malignant brain tumor are about 1%. There's nothing you can do to lower the risk. You can read elsewhere about cancer odds; the most important item is for you not to use tobacco, and to keep up surveillance so that if something does pop up, we can manage it. A majority of cancers are cured nowadays.