I don't think they can deny you if you choose original Medicare and a supplement when you first sign up for Medicare at age 65. It's my understanding that if you try to switch from Medicare Advantage (part C) to traditional Medicare (A/B) and a supplement, the supplement is or can be subject to underwriting, which would be difficult or impossible to clear with a cancer diagnosis. As you mention, the treatment and access options offered through traditional Medicare/supplement offer many advantages. I know the monthly premiums can be higher; my supplement premium is higher than my husband's. But we agreed that the security of full coverage outweighed the monthly charge. So far, so good.