Cysts are perhaps the commonest cause of a discrete breast lump. They are distended and involuted fluid-filled breast lobules that occur most frequently in perimenopausal women between the ages of 38-53 as a result of hormonal changes in the breast. Their incidence diminishes with menopause except in women on hormone replacement therapy (HRT).

Most cysts are small and multiple and only picked up on ultrasound. These can be safely left alone and do not usually require any intervention. Larger cysts can present as a visible or palpable smooth discrete lump that is sometimes painful. They have characteristic features on mammography and ultrasound and are treated by needle aspiration, occasionally with ultrasound guidance.

The cyst fluid varies in colour from pale yellow to brown or dark green and unless blood-stained is usually discarded. If blood-stained or if a residual lump persists after aspiration then further tests would be carried out including fine needle aspiration cytology (FNAC) and/or core biopsy. Very rarely some cancers have a cystic component but this is usually identified on ultrasound and biopsy. Breast cysts are very common and do not confer a significantly increased risk of breast cancer.