Carter likely never had brain cancer. We only have his claim that he had brain cancer and that he was cured of it. He presented no medical evidence that he had brain cancer and was then cured of it.
Where's the evidence that he has a brain?
metastatic melanoma which is what he had can in a not insignificant number of cases can be “cured” or at least long term survival can be achieved if one responds to a classic drug called immune checkpoint inhibitors.