Fibroadenoma (breast)

Last revised by Daniel J Bell on 12 Dec 2022

Fibroadenoma is a common benign breast lesion and results from the excess proliferation of connective tissue. Fibroadenomas characteristically contain both stromal and epithelial cells. 

They usually occur in women between the ages of 10 and 40 years. It is the most common solid breast mass in women of all ages and the most common breast mass in the adolescent and young adult population 1,3. Their peak incidence is between 25 and 40 years. The incidence decreases after 40 years 4.

The typical presentation is in a woman of reproductive age with a mobile palpable breast lump. Due to their hormonal sensitivity, fibroadenomas commonly enlarge during pregnancy and involute at menopause. Hence, they rarely present after the age of 40 years. The lesions are well-defined and well-circumscribed clinically and the overlying skin is normal. The lesions are not fixed to the surrounding parenchyma and slip around under the palpating fingers, hence the colloquial term a breast "mouse".

A fibroadenoma is a type of adenomatous breast lesion. It contains epithelium and has minimal malignant potential 8. Multiple fibroadenomas occur in 10-15% of patients. Patients with multiple fibroadenomas tend to have a strong family history of these tumors.

They are assumed to be aberrations of normal breast development (ANDI) or the product of hyperplastic processes, rather than true neoplasms. Fibroadenomas can be stimulated by estrogen and progesterone. Some fibroadenomas also have receptors and respond to growth hormone and epidermal growth factor.

When found in an adolescent girl, the term juvenile fibroadenoma is more appropriate.

Although they can be located anywhere in the breast, there may be a predilection for the upper outer quadrant.

Fibroadenomas have a spectrum of features from the well-circumscribed discrete oval mass hypo- or isodense to the breast glandular tissue, to a mass with macrolobulation or partially obscured margin. Involuting fibroadenomas in older, typically postmenopausal patients may contain calcification, often producing the classic, coarse popcorn calcification appearance. In some cases, the whole lesion is calcified. Calcification may also present as crushed stone-like microcalcification which makes differentiation from malignancy difficult. 

Typically seen as a well-circumscribed, round to ovoid, or macrolobulated mass with generally uniform hypoechogenicity. Intralesional sonographically detectable calcification may be seen in ~10% of cases 2. Sometimes a thin echogenic rim (pseudocapsule) may be seen sonographically.

  • T1: typically hypointense or isointense compared with adjacent breast tissue

  • T2: can be hypo- or hyperintense

  • T1 C+ (Gd): can be variable but a majority will show slow initial contrast enhancement followed by a persistent delayed phase (type I enhancement curve); non-enhancing internal septations may be seen

These lesions are easily biopsied under ultrasound guidance. When a lesion has the typical features of a fibroadenoma on ultrasound and there are no clinical red flags they can be safely followed clinically. When lesions enlarge or have atypical imaging findings, ultrasound-guided core biopsy is a minimally invasive outpatient procedure that will give a diagnosis with virtually no complications.

Depending on where you work, there may be a maximum diameter above which a biopsy should be done if no previous imaging is available. There is significant local variation in this regard. The reason for intervention based on size is that a phyllodes tumor may be indistinguishable from a fibroadenoma on ultrasound. A maximum diameter of 2.5 cm may be a useful benchmark for biopsy if you have no previous imaging. Interval enlargement is an indication for biopsy.

They are benign lesions with minimal or no malignant potential. The risk of malignant transformation is extremely low and has been reported to range around 0.01-0.3%.

Indications for biopsy include:

  • enlarging lesion

  • atypical findings on ultrasound

  • a lesion above 2.5 cm and there are no previous studies for comparison

  • patient peace of mind: some patients are simply not happy with a palpable mass in the breast without a histological diagnosis; this is a valid and reasonable indication for a biopsy

On a mammogram, fibroadenomas with evolving calcifications may present microcalcifications (punctate, linear and pleomorphic) which can make differentiation from malignancy difficult. In such a case, a fibroadenoma can be considered only if these calcifications are found within a circumscribed mass or at least one additional shell-like calcification is present. Otherwise, a biopsy is warranted 10.

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Cases and figures

  • Case 1
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  • Case 2: on DCE MRI
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  • Case 3: involuted calcific fibroadenoma
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  • Case 4
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  • Case 5
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  • Case 6
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  • Case 7
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  • Case 8: involuted fibroadenomas
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  • Case 9: involuting fibroadenomas
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