"I hope by sharing this it will help others find comfort, inspiration and support on their own journey," she said.
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She said she and her sister did genetic testing in an effort to be proactive about her health. Both tested negative for 90 different cancer genes.
SEE MORE: John Mulaney, Oliva Munn share photo of their new baby
She also had a normal mammogram, but was diagnosed with Luminal B breast cancer in both breasts in 2023. The cancer is aggressive and fast-moving, she said.
Munn said her doctor decided to calculate her Breast Cancer Risk Assessment Score, which led to an MRI, ultrasound, biopsy and eventually double mastectomy.
She said in the past 10 months, she's had four surgeries, and tended to let people see her when she had energy.
What is Luminal B breast cancer? Unpacking Olivia Munn's diagnosis
"I've kept the diagnosis and the worry and the recovery and the pain medicine and the paper gowns private. I needed to catch my breath and get through some of the hardest parts before sharing," she said. "I'm lucky. We caught it with enough time that I had options."
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Munn appeared on the Oscars red carpet Sunday with John Mulaney.
The two have a son together.