My opinion: Since core needle biopsy is a small piece

of tissue, and there is atypia in the center of it, the logic next step will be most likely (still depends what exact nature of the "atypia" though) an open biopsy (incisional or excisional biopsy) to find out if there is more severe lesion than the core needle biopsy showed. Excisional biopsy means the surgoen will remove the whole lesion as a means for treatment as well as further pathologic diagnosis.