Not everyone with cancer loses weight, rapidly or not.
If for some reason the margins weren't clear, or it had already taken up residence elsewhere in her rather vast amount of real estate, body-wise, the big C could still be there, creeping up on her, without her ever knowing because she refuses to do jack shit related to her weight. But "you're not doctors" she says, and states with authority she is in remission. Who are we to argue. We'll just add cancer to the list of things that could potentially kill her, along with suffocation/apnea, liver disease, a fall, choking, etc.