A Fine needle aspiration is used when lumps are very small or cannot be felt,whilst a core needle biopsy( almost same as FNA) has a larger needle to enable the radiologist to obtain an adequate samples of tissue and can provide a more accurate analysis and diagnosis because it removes tissue rather than just cells.The core needle biopsy may not be very accurate in patients with very small lesions.Your doctor in stating that your lump" looks" like a fibroadenoma ,but she can't be very sure of that unless a biopsy is done.Your radiologist is being vigilant and is taking good care of you.Best wishes.
Hello-
The reason being is they cant be sure it is B9 without a biopsy.The core biopsy
will give a better result than needle biopsy.Good luck
Lisa