(CNN) -- Former "Good Morning America" co-host Joan Lunden revealed on Tuesday that she has been diagnosed with breast cancer.
Posting to fans via Twitter and a letter on her website JoanLunden.com, Lunden said she had received an all-clear two weeks ago after her annual mammogram, but "for women who have dense fibrous breast tissue, as I do, often our doctors will recommend an ultrasound as well."
"My ultrasound that day revealed a tumor in my right breast," she wrote. "After a core biopsy was performed, I heard those words that every woman fears and that I never thought I'd hear: 'you have breast cancer.' "
Lunden, who co-hosted "GMA" from 1980 to 1997, said she initially considered keeping the news private, but decided to go public with her diagnosis.
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"I have decided to talk about my breast cancer because since the moment I took the job at 'Good Morning America' I have lived my life sharing my joys and my disappointments with all of you: my pregnancies, my relationships, my career," she said. "I have shared my journey. So it certainly didn't feel right keeping this part of my journey a secret."
Lunden turned to current "GMA" co-host and breast cancer survivor Robin Roberts to offer her first TV interview about having the disease. The news hit home for the staff, as "GMA" co-host Amy Robach is currently being treated for breast cancer.
Lunden told Roberts that the form of cancer she has is particularly aggressive.
"In the beginning it's such a shock, it's a stunner," Lunden told Roberts. "For me, I'm a health advocate, I'm all over America talking to women saying, 'Make sure you get all of your checkups.' In the beginning you almost feel like 'What did I do wrong?'"
Lunden, whose late father was a cancer surgeon, said she gets mammograms every year. She is now in "warrior mode" learning everything she can about the disease and is currently undergoing chemotherapy, she said.
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