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Former U.S. Sen. Norm Coleman underwent surgery Monday at Mayo Clinic to remove the cancerous part of a lung.

Last August, Coleman learned that the throat and neck cancer he began battling in 2015 had spread to his lungs and was at the most advanced stage. After heavy doses of chemotherapy, Coleman said the tumor was gone.

Still, his doctors had him undergo a program of intensive radiation for five weeks in hopes of crushing the disease.

“But cancer is unrelenting,” Coleman wrote in a Facebook post this month, explaining that a follow-up PET scan showed a spot on his lungs that doctors thought could be either “radiation irritation” or a recurrence of the disease. Another scan five weeks later showed the spot had grown and a biopsy determined that the cancer had returned.

Coleman said Monday’s surgery would remove about a quarter of his lung and cut his lung capacity by 15 to 20%.

“As I joked with a friend this afternoon, it simply means that if I were to run a marathon that at Mile 20 I would start to get winded,” he wrote. “I will leave the marathons to others.”

Amid the humor, Coleman spoke of gratitude for his doctors, the Mayo Clinic, his family and his friends. But he also gave an unvarnished glimpse of cancer’s toll.

“Cancer sucks,” he wrote. “There’s other words that have been used to demonstrate defiance against the beast of cancer that I can’t repeat here but I know I have used — as have others around me who love and support me every single day.”

Coleman said he will not let cancer determine how he lives his life. He said he remains active, including traveling the globe for work as well as fishing and spending time at his northern Minnesota cabin. Late last month, he and others escaped injury when his boat was broadsided on Lake Ada.

The Associated Press and Mary Lynn Smith contributed to this report.