Why Gamma Rays can only if outside the body?

We have this homework in Physics and I'm half-done. It's a thing about which kind of particle must be ingested. Alpha and Beta mustn't be because of their ionizing power, now I'm down to one, which is the Gamma. So I thought the Gamma must be the one ingested. Now this one troubles me. Why must be the Gamma Rays, and why they can only outside the body, wouldn't be there an effect if the cells are d by it if it is ingested?