A cyst is a fluid-filled sac. Breast cysts usually do not contain cancer cells (are benign).
Some women who have lumpy breasts also have benign breast cysts that are surrounded by fibrous tissue. Some cysts cause lumps that can be seen or felt (palpable). A cyst that can be felt may move about under the fingers. Its texture depends on where it is located and how much fluid is in it. It may feel smooth or rubbery. It may be hard or soft.
A cyst may be painful and tender or may be painless. A cyst may get smaller after a menstrual period, or it may persist and grow larger. Some cysts can only be seen on a mammogram (breast X-ray) or with ultrasound.
The exact cause of breast cysts is unknown. They may be linked to the hormones of the menstrual cycle. Breast cysts are rare in women older than 50.