ATLANTA — Former President Jimmy Carter said Thursday that doctors had found cancer on his brain and that he would begin radiation treatment later in the day. Mr. Carter, speaking at a news conference at the Carter Center here, said his health had been under scrutiny since May, when he had a cold while traveling in Guyana. The former president added that the cancer, which he said was melanoma, had been found in his liver, part of which was removed during a procedure on Aug. 3. He described the melanoma on his brain as four “very small spots.”