Squamous cell carcinoma of the breast in the form of an intracystic tumor.

Squamous cell carcinoma of the breast is thought to arise through metaplasia of ductal carcinoma cells. We report a case of pure squamous cell carcinoma of the breast with features of intracystic tumor, which was considered to have arisen from metaplastic squamous epithelial cells lining the cyst wall. A 71-year-old woman presented at our hospital with a 40 x 30-mm mass in the lower outer quadrant of the right breast. Mammography revealed a round, high-density mass, which had a mostly regular but partially irregular margin. Ultrasonography demonstrated a solid tumor with an irregular shape protruding into a cystic space, suspicious of intracystic carcinoma. Aspiration cytology confirmed squamous cell carcinoma. A modified radical mastectomy was performed. Histopathologically, the intracystic tumor was a pure squamous cell carcinoma. The epithelial cells lining the inner cyst wall showed mostly squamous metaplasia, and there was continuity between these cells and the squamous cell carcinoma. 13 months later, the patient is free of disease with no adjuvant therapy.