Cancer in his brain is forcing Jimmy Carter to slow down, but the 90-year-old Nobel Peace Prize winner is insisting on keeping up with some of the humanitarian work that has sustained him since leaving the White House as a one-term president 35 years ago. Hours before receiving radiation targeted at four spots of melanoma in his brain on Thursday, the former president warned his staff and a roomful of reporters that he has no intention of missing The Carter Center’s next progress reports on the eradication of elephantiasis and Guinea worm disease.