I hate to break it to you, but Roundup doesn't really work on poison ivy. It doesn't work well on any of the waxy-leafed plants.
However, Monsanto does have another product for the purpose: Roundup® Poison Ivy.
I've never had cancer, but I also haven't had a bad rash from poison ivy or poison oak since I was 4. I regard both risks as low. I have to admit, though, that I'd rather have a toxic plant rash than cancer.
I use Roundup every year on the New Hampshire gravel driveway. I try to stand upwind of it when I spray.