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Circumscribed fibrocystic mastopathy with formation of an epidermal cyst.

A 45-year-old woman complained of a movable lump in her left breast over a 14-month period. The mass was growing slowly and measured 4 cm. The patient was taking no hormones or medications and had no history of breast cancer, trauma to the breast skin, or previous biopsies. Clinical examination found a movable, round, firm 3 x 4 cm mass at the 6 o’clock position in the left breast. No skin changes or axillary adenopathy were noted. The nipple and the contralateral breast were normal. Mammograms revealed a circumscribed, intermediate density, 4-cm mass with a radiolucent halo (Fig. 1A). The mass abutted against the skin. No associated calcifications were noted, and the overlying skin appeared normal. A sonogram showed a round, well-circumscribed solid mass (Fig. 1B). Differential diagnoses included fibroadenoma, carcinoma, phyllodes tumor, adenosis tumor, circumscribed fibrocystic nodule, and chronic inflammatory mass. A fine-needle aspiration biopsy revealed abundant anucleated squamous cells consistent with an epidermal cyst. Despite the benign diagnosis, the patient requested surgical excision of the mass. During surgery, a firm, gray-to-white, encapsulated nodule and some adherent fatty tissue were removed. Microscopic examination of serial sections revealed a cystic lumen filled with soft, white, cheesy material that proved to be keratin (Fig. 1C). The adjacent breast tissue showed florid ductal proliferation, adenosis, papillomatosis, cystic dilatation, and interstitial fibrosis consistent with fibrocystic mastopathy (Fig. 1C and 1 D).