A. There are only two circumstances when an insurance company decline your claim -
a. You cheated
If there is reason for the insurer to believe that you had prior knowledge of your cancer when you applied for it, and said nothing about it, they can drop you. There is a two-year window at the beginning of the policy that allows them that right. After that two year window, you are covered even you cheated!
b. You didn't pay the premium
If you failed to pay the policy premiums, the insurance company could drop you. But of course in the case of a cancer, every insured would do their best to keep paying the policy premium to prevent a policy lapse.