Gary M.K. Tse, MBBS (HK), FRCPC, FCAP, Dip Am Bd AP, MIAC

The Chinese University of Hong Kong Department of Anatomic and Clinical Pathology Hong Kong SAR

In the breast, sclerosing lesions can be problematic as they may mimic malignancy. The common sclerosing lesions include sclerosing adenosis, radial scar/complex sclerosing lesions, and less commonly infiltrative epitheliosis. In other common lesions, stromal sclerosis may occur, particularly in fibroademomas and papillomas.

Sclerosing adenosis is defined as a lesion with increased number of acini in the ductal lobular unit, with extensive fibrosis that results in compression and distortion of the acinar units. It is a common incidental finding, and is usually asymptomatic and microscopic. Pathologically, the ductal lobular units are distorted by the stromal collagen, and the compression may result in angulation of the ducts, giving rise to a pseudo-infiltrative pattern; but usually the lobular architecture is maintained. There is a thick and intact basement membrane around the epithelial cells. Sclerosing adenosis may show metaplastic changes of the epithelial cells, particularly apocrine metaplasia – ‘apocrine adenosis’. In addition, sclerosing adenosis may show superimposed involvement by atypical duct hyperplasia or ductal carcinoma in situ, and in the latter situation, the pseudo-infiltrative pattern may result in an erroneous diagnosis of invasive carcinoma.

Radial scars / complex sclerosing lesions are proliferative lesions with stellate configuration; and they are also known as radial sclerosing lesions, sclera-elastotic scars, stellate scars, benign sclerosing ductal proliferation, non-encapsulated sclerosing lesions, or indurative mastopathy, Currently the common usage of these two terms is segregated by the lesional size, with radial scars being <=1 cm and complex sclerosing lesions being > 1 cm. These lesions are common in older patients, and may be asymptomatic, but larger lesions tend to present at masses. Pathologically, well-established lesions classically show a radiating stellate configuration (much better appreciated at low magnification), with a central sclerosing focus with fibrosis and elastosis. The fibrotic center, usually devoid of epithelial component, is made up of exclusively elastic and collagen fibers; the collagen is usually hyalinized and eosinophilic; the elastic fibers are pinkish. In early lesions the cellularity is much higher, and the fibrotic central area is not well established, and instead may show abundant central myofibroblasts. The epithelial component forms ducts, with those that are closer to the center being compressed and distorted, and the areas further away from the center, the ducts are expanded and cystically dilated.

The differential diagnoses of this group of lesion include micrograndular adenosis, tubular carcinoma, low grade adenosquamous carcinoma, and low grade fibromatosis like metaplastic carcinoma. Judicious use of immunohistochemistry may be useful to delineate the correct diagnosis.

The management of these lesions is excision. Current evidence suggests that pure radial scars/complex sclerosing lesions without superimposed atypical epithelial changes are benign and do not progress to carcinomas. However, radial scars/complex sclerosing lesions diagnosed on needle biopsy can be associated with invasive carcinoma at the time of complete excision; most of these carcinomas are low grade, and at favorable stage.

Infiltrative epitheliosis is also known as sclerosing adenosis with pseudo-infiltration or sclerosing papillary proliferation. It is characterized by infiltrating duct with solid epithelial proliferation, resembling florid epithelial hyperplasia; and there may be frequent focal squamoid appearance, with scleroelastic change of the stroma within and around the lesion, resembling keloid. By immunohistochemistry, the epithelial cells show staining for HMWCK and variably for ER/PR, but the myoepithelial cell layer can be absent (about 30%) or attenuated (about 50%), thus causing diagnostic dilemma. The identification of the benign nature of the epithelium and the recognition of the stromal changes helps to confirm the diagnosis.

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