The term squamous carcinoma should be used for lesions in which more than 90% of the neoplasm is composed of keratinizing squamous carcinoma or one of its variant forms. Squamous carcinoma of the breast is a metaplastic carcinoma because the mammary glandular epithelium is not normally keratinizing. Benign metaplastic squamous epithelium is a potential precursor of pure squamous carcinoma. Squamous metaplasia occurs in the epithelium of cysts (1), fibroepithelial tumors, papillomas, and duct hyperplasia (Fig. 14.1). Extensive benign squamous metaplasia of duct and lobular epithelium has been described in association with fat necrosis and other lesions (2). When squamous metaplasia occurs in an inflamed cyst, metaplastic epithelium may be embedded in the reactive process, resulting in a pattern that is difficult to distinguish from invasive squamous carcinoma. Epithelium displaced into surrounding tissue by a needle core biopsy performed on benign lesions, such as papillomas or fibroadenomas, sometimes undergoes squamous metaplasia. Minor instances of squamous metaplasia are often found in damaged ducts around a surgical biopsy site, and there is rarely extensive squamous metaplasia in healing biopsy site. Tissue or cells obtained by a needle core biopsy from a biopsy site subjected to irradiation in which there is benign squamous metaplasia may display substantial atypia and suggest carcinoma (3).
No specific clinical features are associated with intraductal or invasive squamous carcinoma of the breast. The tumors have indistinct or partially distinct margins on mammography, but no specific mammographic findings have been described (4,5