Oh dear, my usual standby - Covermore - may not be so good in this case, as one of the conditions of cover is that the original cancer has not extended from one location.
Covermore may still be worth contacting though, as my understanding is they will still cover cancellations, luggage, medical etc for anything completely unrelated. So you would still have some cover. The chance of anything related occurring is usually so small (unless you are travelling for six months or so) that it is indeed really infuriating. Good luck.