I'm sorry to hear about your wife's condition. I have personally been affected by both my mother and a sister-in-law having breast cancer.
Please don't take this as being uncaring, but the nature of the medical problem doesn't matter. My wife fell and broke her leg 3 weeks ago and we are cruising in July. Should Carnival be compassionate for us if we had to cancel? If I canceled right now we'd lose 75% of the total cruise price.
When these things happen the tendency is for people to think they are the only one's with a problem which will cause them to miss a cruise. The fact is the cruise lines handle many cases per day where, for whatever reason, people have to cancel and lose some or all of their cruise fare. I know it sounds cold, but a cruise line can't get into the business of making compassion calls every time someone requests their money back.
I see you are in the UK, and I know you have different rules when it comes to cruise bookings - such as the loss of the deposit.
This is why it is vital to maintain travel insurance when you have booked a cruise or other vacation.