Former U.S. Sen Norm Coleman of Minnesota is undergoing surgery at Mayo Clinic to remove the cancerous part of a lung.

Coleman posted on his Facebook page that his surgery scheduled for Monday, July 15 would remove about a quarter of his lungs and cut his lung capacity by up to 20%.

Last August, Coleman said he was being treated for stage 4 lung cancer. He wrote that after “heavy doses of chemotherapy, the tumor was gone.”

But after he had five weeks of intensive radiation, a follow-up scan found a spot on his lungs, and a biopsy determined that cancer had returned.

Coleman, a Republican, lost a close race to Democrat Al Franken in 2008.