Squamous Cell Carcinoma In Situ of the Breast With the Configuration of an Intraductal Papilloma.

A 64-year-old woman presented to our institution with a palpable and painful left breast mass. She denied any other breast symptoms. Subsequent imaging classified it as a US Breast Imaging-Reporting and Data System (BI-RADS) 4A lesion. A core needle biopsy was performed showing atypical proliferating fragments of squamous epithelium suspicious for malignant neoplasm. An excisional biopsy was recommended. Gross examination showed a well-circumscribed pink soft mass measuring 2.0 x 1.4 x 1.3 cm. The entire lesion was submitted for histologic evaluation, demonstrating a neoplasm with branching stroma and exuberant squamous differentiation. The lesion exhibited obvious cytologic features of malignancy like mitotic figures, prominent nucleoli, irregular nuclei, and multinucleation. Collagen IV stain ruled out invasion. The lesion was finally classified as squamous cell carcinoma (SCC) in situ with the configuration of an intraductal papilloma. The possibility of metastatic disease was suggested. A PET scan was negative, and no other foci of disease were found in the remainder of the specimen. The mass was also independent of nipple and skin. Based on the architectural features, we believe that this is a case of an intraductal papilloma that underwent complete squamous metaplasia with no residual adenomyoepithelial components and transformation into an SCC in situ demonstrated by stains. Papillomas can undergo reactive metaplastic changes, usually benign and in small foci. This is the first reported case of exuberant squamous epithelium that transformed into carcinoma in situ with papillary architecture in the breast.