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Did you prove to them you had not yet been diagnosed?
So did the forms from the doctor make it clear that you simply did not know you had cancer when you booked your trip?
Then they are acting in breach of their agreement with you and their refusal to cover you is groundless. They are just trying to get out of paying you.
I suggest that you, or a lawyer acting for you, send the a letter threatening a lawsuit. Give them a deadline for reimbursing you for the cost of the trip and tell them if they fail to comply you will commence legal proceedings and then will also be seeking interest and your costs.
You can most assuredly sue them in Small Claims Court and you can self-represent. If it gets that far and you find you need some help as you go through the process you can always post on this site again and I can assist you further.