Needle biopsies only collect a small piece of tissue from the lump; surgery may be required to get a better look. Also keep in mind that if your lump is cancerous, the biopsy won't be 100% conclusive about its extent, including the degree of any lymph node involvement. Another reason you might need surgery is if a needle can't reach your lump to take a sample in the first place, such as if the lump is too close to the nipple or chest wall.
On the other hand, chances are that your lump is not cancerous at allthat's the case in a majority of biopsies.
If your lump test does come back negative for cancer, it's important to verify that it stays that way. "If it comes back benign," says Peter M. Jokich, MD, director of the Rush Breast Imaging Center at Rush University Medical Center in Chicago, "we like to watch the site and repeat the mammogram in six months to make sure we didn't miss something or the lump hasn't grown."