What does fine needle biopsy of the breast tell you?

Cancer or not. A fine needle biopsy of the breast is done when there is a lump or bump the surgeon can feel. A hollow needle is put into the lump to get some cells, which are then looked at under the microscope. If there is a cancer, the fine needle biopsy might diagnose it, but it's not 100%. Other things such as fibrocystic breast disease or fibroadenoma (benign lump) can be diagnosed this way.
Whether there. Are malignant or suspicious cells in the aspirate, which would prompt surgical excision. In the US, fine needle aspiration is not commonly performed on breast lesions due to a variety of reasons. Needle core biopsy is generally the standard.
Pap Smear of Breast. Fine needle aspiration (fna) breast biopsies involve drawing cells out of a lump for definitive dx. If cancer cells are seen, it is very reliable; however, a benign result can (rarely) miss a cancer. Therefore, our preference these days is to do a core needle biopsy (+/-vacuum assisted). This removes tissue rather than cells and is more reliable for diagnosis.
Not the whole story. Fine needle aspiration is operator dependent especially in small and deep breast mass. Suspicious breast mass should consider complete excision for pathologic extermination.