The leyline magic system; do you need to be physically on it to cast magic?

So, in the witcher tabletop game it says you need to physically be on the leyline to cast magic. You need to be close to a place of power to cast more stable and powerful versions of the same spells. How does it work in the books? Do you need to be physically on the leyline? Does the spell weaken or become more difficult to control if you are far away from a leyline? or can you not cast spells at all?