Breast cyst. Coloured mammogram (breast X-ray) of a benign cyst (round, centre) in a woman's breast (red) seen from the side. A cyst is an abnormal lump or swelling that is filled with fluid or semi-fluid material. This may be a retention cyst, one that arises from the blockage of a duct in glandular tissue. Cysts that become too large may need to be surgically removed.