First off, mammograms have recently been proven to be less accurate at finding breast cancers than ultrasound, which is what I personally would recommend. A "well-defined" fibroadenoma PROBABLY means that what you have is benign, and the next step is for your doctor to perform a biopsy if he/she feels that it may be otherwise. Do follow up with your doctor soon, as no finding of something that shouldn't be there is to be taken lightly!