Breast fibroadenoma. Light micrograph of a section through a fibroadenoma in breast tissue. Fibroadenomas, also called cystic mammary dysplasia and fibrocystic disease, are the most common type of benign (non-cancerous) tumour of the breast. They occur most frequently in woman of childbearing age, forming a lump 1-5 centimetres in diameter. They can take a number of different forms and can be removed surgically, rarely becoming cancerous. The type shown here is apocrine metaplasia because of the resemblance of the cysts to apocrine sweat glands.