U.S. Senator Roger Marshall (R-Kansas), a mentee of Dole's, said "I haven't known a better public servant" and that he expects the longtime senator to meet the challenge as he's done numerous times throughout his life.
The 97-year-old made the announcement Thursday, saying he would begin treatment on Monday.
Dole, who was seriously wounded during World War II, has suffered a series of health ailments in previous years.
Kansas Gov. Laura Kelly, a Democrat, called Dole a friend and tweeted wishes for a speedy recovery. He ultimately won the Republican nomination but lost the election to incumbent President Bill Clinton.
Retired four-term Kansas Sen.
Dole's involvement in presidential politics is as lengthy as anyone to never hold the office.
He served with Clinton following the September 11, 2001, terrorist attacks as co-chairman of a scholarship fund for the families of the victims.
In 2018, Congress awarded him the Gold Medal. He earned two Purple Hearts and two Bronze Stars for valor during his service.
He served for 27 years as a United States senator from Kansas, including two stints as the Senate majority leader.
Bob Dole campaigns for the U.S. Senate in November 1968.
He spent his first eight years in Washington as a member of the House of Representatives before graduating to the Senate in 1969 and serving through 1996.