being social 101: it's only weird if you make it weird
This is actually super true. I get away with doing stuff other people would get socially lashed for all the time because I don't make it weird. I mean, I wouldn't have done what you did because I know what Leukemia is and I know that it's not loving gangrene and so it by definition has already spread, but if I did write that anyway I'd have two backup plans: cover my ass and make other people seem like the starfishs.
Blame other people for not being able to be lighthearted about a situation where the subject really needs some humor or comfort in their life.
Cover your ass by explaining that cancer can
metastasize and cause tumors in other areas, which is a bad thing and you're essentially wishing them well.