I’m sorry to hear about your Dx.
But as with MS, please don’t blame anything you’ve done!
First off, Rebif is NOT an immunosuppressant anyway. It’s an immune system modifier, which is different.
Secondly, I think the lifetime risk of cancer (any type) is about 1 in 3 anyway, which means it’s sad but true that you don’t need any special risk factors to end up with it. About a third of us will - the vast majority having never taken Rebif - or anything like it.
Finally, I don’t know how long you’ve been on Rebif, but even if it was a risk factor (which I’m sceptical of), I believe most risk factors take years or even decades for the harm to materialise. E.g. people with smoking-related cancer get it after a lifetime - NOT after their first few months of ciggies.
So unless you were one of the very earliest to have been offered Rebif, I wouldn’t have thought it would have had time, yet, to do what you suspect of it.
I’m sure it’s just coincidence. You’ve got enough on your plate, without thinking you somehow caused it. I’m sure you didn’t!