I think you will find - as I have - that there is no such thing as cheap insurance when you have a pre-existing condition. However what you can do is take out insurance that excludes your pre-existing condition - ie you would be covered for anything else like a broken leg, heart attack etc etc as long as it was nothing to do with the leukaemia. That way you can get fairly cheap insurance.
I have Emphysema and excluded that from my policy as I did not believe that it would cause me to be hospitalised. However when I had a fall in Las Vegas and incurred $5000 expenses in X-rays, CAT scan etc the insurance paid up for this.