( 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 . `` 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 did n'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 .
Lunden announced her diagnosis on television . A mammogram did n't reveal the diagnosis . She said she 's now in ` ` " mode ''