A fibroadenoma is a painless, unilateral, benign (non-cancerous) breast tumor that is a solid, not fluid-filled, lump. It occurs most commonly in women between the age of 14 to 35 years but can be found at any age. Fibroadenomas shrink after menopause, and therefore, are less common in post-menopausal women. Fibroadenomas are often referred to as a breast mouse due to their high mobility. Fibroadenomas are a marble-like mass comprising both epithelial and stromal tissues located under the skin of the breast. These firm, rubbery masses with regular borders are often variable in size.

Types

Complex Fibroadenoma

Rapidly growing hyperplastic cells; the pathologist can establish diagnosis after carefully reviewing the biopsy of breast tissue

Juvenile Fibroadenoma[1][2]

This fibroadenoma is commonly found in patients age 10 to 18 years. These fibroadenomas have a faster growth rate and eventually shrink or disappear.

Giant Fibroadenoma

This non-cancerous lesion is slightly larger than other types. It commonly needs excision because it compresses or replaces normal breast tissue.

Phyllodes Tumor

A stromal tumor of the breast that can be either benign or malignant. It is not a fibroadenoma, therefore; it needs to be watched closely. Most doctors recommend removing it.

Causes of fibroadenoma are debatable, but professionals believe that the lesion has a hormonal etiology related to the increased sensitivity of breast tissue to the female reproductive hormone estrogen. Fibroadenoma usually grows during pregnancy and tends to shrink during menopause. This supports the hormonal etiologic theory. Females who take oral contraceptives before 20 years of age tend to suffer from fibroadenoma at higher rates than the general population.

Fibroadenoma tends to occur in early age. It is most commonly found in adolescents and less commonly found in postmenopausal women. The incidence of fibroadenoma decreases with increasing age and generally found before 30 years of age in females in the general population. It is estimated that 10% of the world's female population suffers from fibroadenoma once in a lifetime.

Hormonal

Fibroadenoma arises from stromal and epithelial connective tissue cells that are functionally and mechanically important in the breast. These tissues contain receptors for both estrogen and progesterone. For this reason, fibroadenomas tend to proliferate during pregnancy because of excessive production of female reproductive hormones. Hormone sensitivity causes excessive proliferation of breast connective tissues.

Genetics

Mediator complex subunit 12 (MED12) gene is also important in the pathophysiology of fibroadenomas.

The histological diagnostic features of fibroadenoma can be described as sheets of uniformly distributed epithelial cells that are typically arranged in a honeycomb pattern. There is a presence of foam cells and apocrine cells and an absence of excessive mitotic activity or anaplasia. Calcification may also be present. Juvenile fibroadenoma has increased in stromal cellularity. The stroma is hypovascular as compared to the malignant neoplasms. The basement membrane also remains intact in fibroadenoma which reflects its benign feature.Microscopic Features

Contains both stromal and epithelial tissues that are arranged in 2 patterns:

1. Pericanalicular: Includes the proliferation of stromal cells around epithelial structures
2. Intracanaliccular: Includes the proliferation of stromal cells compressing epithelial cells into clefts

Unique Histological Features of Various Fibroadenomas

1. Complex fibroadenoma: Sclerosing adenosis, calcification, apocrine metaplasia.
2. Juvenile fibroadenoma: Epithelial and stromal hyperplasia, absence of mitoses, thin micropapillae
3. Myxoid fibroadenoma: Stroma has myxoid features