A fine needle aspiration biopsy (FNAB), is a very thin needle attached to a syringe to withdraw (aspirate) a small amount of tissue from the suspicious area. This tissue is then looked at under a microscope. The needle used for FNAB is thinner than the ones used for blood tests.

If the area to be biopsied can be felt, the doctor locates the lump or suspicious area and guides the needle there. If the lump can’t be felt, the doctor might use ultrasound to watch the needle on a screen as it moves toward and into the mass. (This is called an ultrasound-guided biopsy.) Or, the doctor may use a method called stereotactic needle biopsy to guide the needle. For a stereotactic needle biopsy, computers map the exact location of the mass using mammograms taken from 2 angles. This helps the doctor guide the needle to the right spot.

The doctor may or may not use a numbing medicine (called a local anesthetic). The needle used for the biopsy is so thin that getting the medicine may hurt more than the biopsy itself.

Once the needle is in place, fluid or tissue is drawn out. If clear fluid is withdrawn, the lump is more likely a benign cyst (not cancer). Bloody or cloudy fluid can mean either a benign cyst or, less often, cancer. If the lump is solid, small pieces of tissue are drawn out. A pathologist (a doctor who is expert in diagnosing disease from tissue samples) will look at the biopsy tissue or fluid under a microscope to find out if it’s cancer.

A fine needle aspiration biopsy can sometimes miss cancer if the needle does not get a tissue sample from the area of cancer cells. If it does not give a clear diagnosis, or your doctor still has concerns, a second biopsy or a different type of biopsy should be done.

Adapted from the American Cancer Society