Squamous cell carcinoma of the breast.

We saw two cases of pure squamous cell carcinoma of the breast, one of which is associated with dermatomyositis. Electron microscopy of appropriately fixed tissue obtained from one tumor confirmed the squamous nature of the polyhedral and spindle-shaped tumor cells. One tumor was assayed for estrogen and progesterone receptor proteins and was found to be lacking both. Test results for mucin and carcinoembryonic antigen (CEA) were negative in both tumors. In contrast, a similarly examined case of invasive ductal carcinoma with areas of squamous metaplasia had slightly elevated concentrations of estrogen and progesterone receptor proteins, and test results for mucin and CEA were positive.