Former US senator George Mitchell, a peace envoy in Northern Ireland in the 1990s, is starting treatment for leukaemia at a Boston hospital a day after his 87th birthday.
An email outlining the diagnosis and treatment from the president and chief executive of the Senator George J Mitchell Scholarship Research Institute was obtained by the Portland Press Herald.
In the email to the staff and board, Meg Baxter wrote that the goal of treatment at the Dana-Farber Cancer Institute is to achieve remission, the newspaper reported.
Mr Mitchell, also a peace envoy in the Middle East focused on the Palestinian-Israeli conflict during President Barack Obama's administration, is a consultant and former chairman of the global law firm DLA Piper.
He brokered the historic 1998 Good Friday Agreement in Northern Ireland and was awarded the Presidential Medal of Freedom in 1999.
Mr Mitchell was also selected by Major League Baseball to investigate steroid use by players, leading to the eponymous Mitchell Report.
The Democrat won election twice as a US senator and served as majority leader after first being appointed to fill a position left vacant when the late Edmund Muskie was chosen to serve as secretary of state under President Jimmy Carter.
Before that, he served as a US attorney and federal judge.
Mr Mitchell was previously diagnosed with what was termed "small, low-grade and localised" prostate cancer in 2007.
