Chicago Teachers Union president Karen Lewis, who had a tumor removed in October 2014 when she was diagnosed with an aggressive form of brain cancer, was undergoing brain surgery Wednesday, a source close to Lewis said.

“Cancer may be back,” the source said.

Chicago Teachers Union Vice President Jesse Sharkey, in a statement, said he talked to Lewis on Tuesday. He said she was in “great spirits” and was planning on undergoing a “medical procedure.”

“Karen remains in regular contact with me and the rest of our union colleagues at the CTU, and to be honest, was more interested in directing a message to the CTU members, parents, students and people of this city. Karen said, and I quote – ‘Tell our delegates let’s get ready to fight!'” Sharkey said.

“She said for us to go back to our school communities tomorrow ready to fight for safer, well-resourced and staffed school communities and for a better contract for our members.

“As we all know, Karen is a fighter. She is looking forward to the coming rallies, actions and negotiations, and is excited about attending the AFT convention in Pittsburgh this summer.”

Lewis returned to work after her previous surgery but suffered a stroke last year in an area of the brain where the tumor was removed.

At the time, she called the stroke “a minor setback in my recovery from brain cancer.”

Lewis’ 2014 cancer diagnosis derailed a likely run for mayor. Her supporters were collecting signatures to put her on the ballot when she underwent surgery. She ended up supporting  Jesus “Chuy” Garcia, who lost after forcing Mayor Rahm Emanuel into a runoff.

The type of cancer that Lewis was diagnosed with — glioblastoma — is one of the deadliest cancers, typically proving fatal within less than a year and a half.