CHICAGO (CBS) — Karen Lewis, who has led the Chicago Teachers’ Union through some tumultuous negotiations with the city, is retiring.

The CTU confirmed Lewis has filed her retirement papers. She underwent a medical procedure earlier this month, and has been battling health issues for several years.

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Lewis was diagnosed with an aggressive form of brain cancer in 2014.

In October, Lewis suffered a stroke in the same area of the brain where she had a tumor removed in 2014.

Lewis’ brain cancer diagnosis derailed plans to run against Mayor Emanuel in 2015. She ended up backing Cook County Commissioner Jesus “Chuy” Garcia, who forced Emanuel into a runoff, the first ever in a Chicago mayor’s race.

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Lewis led the union during the week-long 2012 teachers strike. The union and school board narrowly avoided another walkout in 2016.

“I’ve seen firsthand the tenacity and drive that makes Karen Lewis a worthy advocate for Chicago children and teachers, and we’ve grown to admire each other as a friends,” said Emanuel. “Karen may be stepping down from her position at CTU, but I know she’ll never stop fighting for Chicago’s children.”

Chicago Public Schools CEO Janice Jackson said in a statement:

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“Karen Lewis has been a tireless advocate for Chicago’s educators and students, and she’s someone I’ve been honored to call a partner and a friend for many years. I want to thank Karen for her commitment to promoting quality instruction, and I know she will remain an important voice for our schools going forward.”