Dear worrywart1031: Cysts are, by definition, benign. Some types of complex cysts may have solid components and these can rarely be associated with malignancy. Unilateral dilated ducts on mammography are most often due to benign processes but may be indications for biopsy if the finding is new or progressive, in a nonsubareolar location, or associated with indeterminate calcifications. Without reviewing your ultrasound and conducting a history and physical, it is impossible to make a specific recommendation. Your surgeon will likely be better able to review all the data and make recommendations as they pertain to you.