Hi, I think you should accept but tell them about your cancer diagnosis. It sounds like a great opportunity and hopefully your cancer will turn out to be early stage as mine was, I needed one week off work when I had the lumpectomy but only radiotherapy after that (no chemo)  and managed to get mostly early appointments so only missed a small amount of work each day (fifteen sessions). I'm sure they'll be understanding, don't turn it down without talking to them first.  Good luck! 

Thanks so much, you're right, it makes sense to talk to them!