Joan Lunden, former 'GMA' host, has cancer

(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.