I was wondering the same thing. Chemo would be next to impossible to hide if you have any relationship with the person at all.
They've already said that Ray Romano will be on the show for something like 12 or 13 episodes and it will get to the point where Sarah has to make a choice between him and Mark. It was a bit of foreshadowing when Camille let Mark be in the family picture. I guess I just don't like the triangle aspect of the whole thing.