Cancer history not enough to get an authorization number

Ive been trying to get an authorization number for a scan at bankmed since mid Feb. They said they needed a motivation letter from my specialist, which he did write and send but they said it was not enough they needed more info on why I had to get a mammogram at my age. The reason for that is: I was diagnosed with stage 2 cancer last year and was on treatment from November till January this yr. It appears on their records. The doctor wants me to do that scan to see if I have anything on my breasts too. That is not enough for them. Im so disappointed cz I once did the mammogram years ago and never went through such trouble.