Actually, Rose never says the word "cancer" in the whole episode (S.O.S.), or any since. It was just infered from the fact that she talks about a terminal illness and the word "remission" (which almost always refers to cancer, but occasionally some chronic autoimmune and infectious diseases).
What's actually said:
ROSE: Bernard, I'm dying. I'm sick, and I'm dying. I've been in remission for the last couple of years. But now it's back and this time it's not going away. My doctor says I have about a year -- maybe a little longer.
I suppose anything could be possible for the Lost writers to write in, but if they are medically accurate, it's probably not Rose's. Mostly because Jack is a doctor used to reading X-rays, and experienced docs and radiology technicians can tell a woman from a man by the breast shadow, which shows up in a standard CXR.
