Hamartomas are rare and present as well-circumscribed, mobile lesions.

They are often impalpable and may be an incidental finding on screening mammography.

Breast hamartomas are one of the few breast abnormalities that have characteristic features that allow confident diagnosis without resort to needle biopsy or surgical excision.

Hamartomas appear as well-defined lesions containing areas of soft-tissue and fat density - so called “breast within a breast” appearance.

There is usually a surrounding capsule present indicated by a thin soft-tissue line demarcated by fat on either side.

The lesion may be harder to diagnose on ultrasound than with mammography because it tends to blend into the background appearance of the breast. Hamartomas have a well-defined margin and contain varying amounts of hypoechoic fat and echogenic breast tissue.

Macroscopic Appearance
These lesions have a wide size range, with diameters of up to 25 cm having been recorded. The majority, however, are under 4 cm in diameter.

They are, like fibroadenomas, well-circumscribed, firm, round or oval masses.

The cut surface has a soft or firm consistency and a color that varies from gray to yellow. The consistency and color is dependent on the amount of fat present in the lesion. The lobulation and ductal clefts that characterize fibroadenomas are absent in these lesions. Small cysts are a frequent finding.

Microscopic Appearance
Hamartomas are well delineated from the surrounding breast tissue.

They comprise a mixture of breast lobules, connective tissue and fat. The breast lobules may be discrete and histologically normal. Frequently, they lack specialized lobular stroma and may merge to form very large units.

Occasionally, ectatic ducts and apocrine metaplasia are seen; however, epithelial hyperplasia is not a feature. The connective tissue stroma is sparsely cellular and densely fibrous. In most cases, the stroma includes areas of pseudoangiomatous hyperplasia. This refers to anastomosing artefactual clefts in the stroma lined by flattened stromal cells. Varying amounts of mature adipose tissue are incorporated into the connective-tissue component.

Occasionally, smooth muscle can be demonstrated in these lesions using smooth-muscle actin immunohistochemistry.

These lesions are best left alone.

A.D. Purushotham, P. Britton and L. Bobrow
A prospective study of benign breast disease and the risk of breast cancer. JAMA 2002


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