A man walks through former President Jimmy Carters' campaign headquarters in downtown Plains, Ga., Sunday, Dec. 13, 2015.  A recent MRI showing no cancer on Jimmy Carter's brain is &quot;very positive&quot; news for the former president but will not end his medical treatment, doctors said. Carter, 91, announced Dec. 6 that doctors found no evidence of the four lesions discovered on his brain this summer and no signs of new cancer growth. (AP Photo/Branden Camp)