"In some cases, a small sterile clip will be placed into the biopsy site of the breast to mark the location in case a future biopsy is needed. This microclip is left inside the breast and causes no pain, disfigurement, or harm to the patient."
And .. it's super tiny, flat, and only about the size of the tip of a pencil. I have a lot more metal in my ankle than I will ever have in my breast. No worries!