ROCHESTER, Minn. — Former St. Paul Mayor and U.S. Senator Norm Coleman will have surgery at the Mayo Clinic Monday to remove part of a cancerous lung. 

Coleman posted on his Facebook page earlier this month that his cancer, which he refers to as "the beast," had returned. During a regularly scheduled PET scan six months ago, doctors ago spotted a cancerous lymph node on his lung which turned out to be stage 4 metastatic cancer. Heavy chemotherapy shrunk the tumor and Coleman thought he was out of the woods. 

In an attempt to insure the cancer was beaten back, Coleman then embarked on an intensive, five days a week radiation program for five weeks. 

RELATED: Ex-Sen. Coleman to have part of lungs removed

"My hope was that we had put a knife through the heart of the beast. But cancer is unrelenting," Coleman wrote on his Facebook page. "Following the radiation protocol, six weeks ago I had a follow up PET scan-in my mind just to confirm that the beast had been vanquished. To my horror, the scan showed a spot on my lungs-what my medical chart labeled “suspicious recurrent disease. Another PET scan, another battle to fight."

Coleman's doctors told him the cancer in his lungs has made itself immune from the effects of previous treatments, so surgery was the best option on the table. They will remove the portions of his lungs that show cancerous growths. 

"Bev Coleman didn’t raise her kids to be pushovers and I’m not intending to give any quarter in this war against the beast," the former mayor wrote. "I will, however, give up a quarter of my lungs. On July 15 I will undergo a video-assisted thoracoscopic left lower lobectomy- which, according to my doctor will deprive me of about 15-20% of my lung capacity. 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."

"I will leave the marathons to others," he added.

Coleman says he is thankful for a family that loves him, friends that sustain him, and for the host of "prayer warriors" in his corner. 

"To the beast I say this to you: My war against cancer isn’t won. Yet."

RELATED: Norm Coleman survives being hit by boat

Coleman has been fighting cancer since 2015, when it first appeared in his throat and neck. Last month he survived a close call when an inattentive fisherman ran his boat into one Coleman was fishing out of. Coleman saw the boat approaching at the last instant and dove into the water of a northern Minnesota lake. He said the incident underlines the importance of wearing a life jacket.