Blume, 74, was diagnosed June 12 following a routine ultrasound of her dense breast tissue, the New York Daily News reported.
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The author of "Are You There, God? It's Me, Margaret," "Blubber," "Superfudge" and other classics told fans she is "doing fine" and now recovering in New York following a mastectomy a month ago.
She described the shock of the diagnosis, writing that her family has no history of the disease.
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"I've never smoked, I exercise every day, forget alcohol - it's bad for my reflux - I've been the same weight my whole adult life.How is this possible? Well, guess what - it's possible," Blume wrote.
Seventeen years ago, Blume underwent a hysterectomy after being diagnosed with cervical cancer but "caught it just in time," according to People magazine.
A New Jersey native, Blume's books have sold more than 80 million copies and been translated into 31 languages, according to her website.
She wrote that she's considering getting back into writing after Labor Day, "kind of like starting school."