I'm going to lie to break up with my boyfriend.
What lie are you going to tell him?
I'm gonna tell him I have cancer.
It's perfectly fine to break up with someone if you no longer want to be with them. However, lying to them about why you're breaking up, especially if you are claiming to have cancer, is not a good way to do it. When they find out you lied about the cancer they will be even more hurt and upset.
You're assuming that I actually care about the feelings of my boyfriend.
I don't have all the details of the story so I don't know if he did something to upset you. Even if he did though, you should still try to be respectful in breaking up with him. There's no need to lie since that will just make things more difficult between you two.