Coleman, 66, announced his diagnosis Monday morning on his personal Facebook page, according to multiple media sources. He said the cancer is isolated to a portion of his neck and tonsil.

"It is clear that my cancer, while serious, is very treatable and the prognosis is extremely positive," he wrote.

Coleman represented Minnesota for one term in the Senate from 2002-2008 before losing his reelection bid to Sen. Al Franken by just 312 votes. Prior to his time in Congress, he served as St. Paul's mayor for eight years.